posted 08-28-2013 01:39 AM
All good points above. All the visors look very similar, basically since the helmets in question are about the same size and shape. It is difficult to tell them apart, so it is the small details that help identify them.
Good point on the Mercury helmet. The earlier Mercury helmets used a pneumatic visor seal so the visor could rotate down closed and the visor seal inflated outward to the visor. In the closed position, the visor cutout rested on top of the edge of the O2 connection boss, so the O2 connection boss did not interfer visor seal as it inflated.
The Cooper helmet for the advanced Mercury spacesuit used a mechanical visor seal, so the visor rotated down, and then as the bailer bar was lowered a cam mechanism pushed the visor inward toward the helmet shell, sealing the visor against the seal on the helmet. Since the O2 connection boss provided an opportunity for the visor the hang up on it as the visor translated inward to the helmet shell, it was moved to the right rear of the helmet away from the visor. Thus no cutout was needed in the visor frame.
However the biggest difference with the visor for the Cooper helmet was the actual pivot point was not in the fiberglas of the visor but in mechanical linkage at the edge of the visor. So the plexiglas and fiberglas part of the visor did not extend around the pivot location for the Cooper helmet visor.
Gemini helmets are somewhat similar in shape and size to the HGK-13 (same company, same vintage) so the visors look very similar, however the sealing methods are different so the visor pivot mechanisms are accordingly different. Although the HGK-13 helmet which was sold in the Heritage auction was really not an X-15 helmet, the clear visor is the same, as well as the helmet shell.
The "X" on the visor is part of the part number, and is the same on all the ones which have surfaced, and matches the ones on production USAF helmets. The companion tinted visor has a similar part number. This spare visor is serial no 447, so a lot of these were made, most of the ones which show up still have the protective coating on the visor.
This is all probably too much information but perhaps someone may find it useful.
Regardless of the specific helmet, these are great visors for any collection.