Just 300 people in the U.S. were selected to participate in the Steam Machines beta, which shipped boxes to folks beginning on Friday. They know which number they are in the beta by looking at the row of 300 ventilation holes on the top of the unit and finding the position of the single countersunk hole on theirs.
One lucky participant wants to find all 299 of his comrades, and has set up this page to identify them as they upload videos and images of their units. Steam user Cynagen has located just three of them so far -- Nos. 39, 53 and 54. (He has yet to receive his machine, and does not know his number yet) The page he's set up is a grid of 10 rows of 30 holes, and as he spies new unboxings (or has them sent in) he fills it in with a link to the video or images. The grid represents the top of the box (or side, in tower formation), with its USB ports located on the side of the unit at the top left corner.
Redditor coelbehr's unit (above), for example, is No. 53. Some who have received their beta unit, like the guy in the unboxing video we showed yesterday, may have not reported their number yet (or displayed it in a way where the countersunk hole's position is clearly visible). Cynagen is hoping to "collect" everyone who has, so if you're one of the fortunate 300, check in with him. And if you're a spectator, see the page for more photos of the unit from other users.