I see this asked fairly often, so I figure it time to make an "official" post to put this question to bed once and for all.
So you think you have a 1st run Peak or you are looking to buy a Peak someone is claiming to be a 1st run. Boy it would be nice to know for sure. While it is true that 1st run Peaks weigh different from 2nd run Peaks, I wouldn't rely on that. You may not own a good scale. Even if you do, if the Peak is on BST, you can't weigh it. More importantly depending on if it's raw, ano'ed, or painted, it will weigh different. I have a very nice digital scale and have weighed all my Peaks and the numbers are all over the place. I even have 2nd run Peaks that weigh as much as gram different from one another.
What you really want to do is be able to visually identify a 1st run Peak, and thankfully this is possible.
The proto and 1st run Peaks came from Chris' first machining shop. The 2nd run Peaks came from Chris' second machining shop. (see info below). This second machining shop does a tool change which results in a machining line on the Peak. If your Peak has this line, it is a 2nd run... period.
Here you can see the tool change machining line inside the hub of this 2nd run Peak:

Note that this 1st run Peak does not have that machining line:

OK. So you have a Peak without the machining line. That alone isn't enough for me to be 100% certain that it's a 1st run. If it's painted, the paint may be thick enough to hide the line. Or someone (like myself) may have taken a raw 2nd run Peak, sanded down the line and polished it.
Here is my 2nd run Levi painted Atari Peak which has the machining line but the paint is thick enough such that you can't see it:

Here is my 2nd run raw which I sanded (thus removing the machining line) and polished:

So weights vary and machining lines can be removed. We need a definitive way to identify a 1st run Peak, and luckily we have one. The outer edge/lip of the rim is more rounded on a 1st run Peak whereas the outer edge/lip of the rim on a 2nd run Peak is sharper; less rounded. Unfortunately the difference is so subtle I still have to set two Peaks side by side to see the difference.
Here are two 1st run Peaks side by side (red Yoyowiki and 28S):

Note how the outer edge/lip of the rim is rounded, leaving a nice "v" shaped white space between the outer edge of the rims of the two Peaks.
Here are two 2nd run Peaks side by side (raspberry maple drip and A-grade pink/purple wash):

Note that the "v" shaped white space between the outer edge of the rims is much smaller because the outer edge of the rim is not rounded off as much as the 1st run.
Another view:

So if you think you have a 1st run Peak find a 2nd run Peak and put them side by side. Here's what you want to see:

Note how the outer edge/lip of the 28S's rim is clearly more rounded off.
While on the subject, here's a bunch of useful information which has all been posted before, but would be helpful to have here as well:
Okay here's the Peak history to clear a few things up:
My friend's machine shop
- 6 prototypes
1st Machine shop = 1st Gen
- 1st 50 Painted Peaks
- 1st run 28 Stories Peaks (Fall 2007 - had the flat spikes on some of them - some had vibe)
- Some random anos, raws and the Yoyowiki Peak
- Most were given to contests, friends, team members
2nd Machine shop = 2nd Gen (he also runs bvms & bassalopes)
- Custom Painted Peaks Levi did in 2008 that sold on ebay
- Atari Peak, Iron Maiden, Tree, Gold sparkle, Plaid...
- all anodized Peaks from this year
- a couple raw Peaks that went to contests
3rd machine shop runs only Woolys and a new prototype we're working on
More details about the 6 proto Peaks:
1 white - went to Boyd
1 grey -0003, Elephark has it
1 raw - Dremeled/chipped away silicone groove, went from Chris to Elephark to Vendetta, 1st Peak to be machined, used and heavily tested by Chris, who did the Dremel "mod"
1 raw - once owned by G-String Paul, sold to someone in Brazil, may have found its way back to North America but current owner wishes to remain anonymous
1 red - 0001 painted, Chris kept this
1 grey/ice blue - 0002 painted, went from Chris to Vendetta to me (Ben)
Pics of painted proto 0002 which I own:


Pic of 0003:

More details about the 50 1st run numbered painted Peaks:
Here's the work that went into those 1st 50:
16 hours prep/paint
8 hours siliconing
4 hours assembling (had to match halves)
3 hours packaging
+ the time it took Paul to make the custom string
+ the endless amount of time online we spent keeping everyone updated with our progress
+ the fun we had with starting a yoyo company
This was the 1st yoyo Caribou Lodge released. It was made in 2006 and is still the preferred yoyo Alex Berenguel is using for contests.
Under "1st machine shop" Chris references "some random anos". This references the Peaks Chris used to test finishes on. I know of 6, five of which are in this pic:

See-Bass has the green splash Peak
The 2 blue splash (i.e. Team Edition) Peaks are owned by Chris (c-foam) and Attybud
The black/blue acid wash Peak belongs to "HVizier"
The green/blue acid wash belong to "Elephark "
The 6th I know of is the "Boyd" Peak. Half green splash / half brown and blue wash. Currently owned by Trevlor:

Also under "1st machine shop" Chris references the "Yoyowiki" Peak, which is anodized in flat red, which I own and is in some of the comparison pics above.
Just to reiterate, the only 1st run Peaks Levi painted are the numbered ones (unless someone got a raw 1st run and sent it to Levi to a custom job, which at this point in time I don't believe has happened). I can't tell you how many times I've seen a Levi custom painted Peak on Ebay listed as "1st run" and the seller is simply wrong.
Chris references under "1st machine shop" "some random raws"... I have two of these, one which Higby painted, one which I had custom ano finished. It is really only these "random raws" which could be causing any confusion about "is this a 1st run or 2nd run?" All protos are known and mostly accounted for. All six 1st run Peaks with unique ano jobs which Chris had done are known and accounted for. All Levi numbered painteds are easily identifiable, as are all 1st run 28S's.
- Ben