
I agree with Chris and Bill - The NASA standard hesalite crystal, solid case back manually wound and for me, on a black alligator.That said, I own a bomb-proof Mark II which has just returned from Omega re-finished and new glass which looks fantastic and is a joy to own. The upshot of what I am saying is - get any, they're all great!!!
Cheers - David

I know and it is true the "original is original" although the cal is not 861 (nor 321 :0) ) any more but 1861 ......
but is so damn attractive to have transparent case back and enjoy the cal 1863 .....
I'm also torn between the one with solid (approved by NASA) and one with Transparent (not approved by NASA)
so basically there are two options in my opinion if you are after Speedy Pro
Reference
3573.50.00 (cal 1863, transparent case back, sapphire crystal, not approved by NASA)
and reference
3570.50.00 (cal. 1861, solid case back, hesalite crystal, approved by NASA)
basically if you enjoy like I do most of all seeing cal and playing with chrono while looking at it go for first
if you wanna something closest to one that was on Moon as you can go for second
Best regards
Damian