You are correct. The ones from the 60's have the swoop brace. It is NOT quite the same brace as the older 10M necks..but at a glance, it's close.
The ones from the '50's do not have that, as you observed. And the '50's ones have the circular ring at the base of the octave key (like an old 10M) while the later ones have the octave key ring which 'splits' or 'forks' off of the key spine.
They are, however...interchangeable...you can put a newer 16M neck on an older model and vice versa...you can even put a 16M neck on a 10M (the kinds sans the double-socket of course) and have it work quite fine. Same bore, length, and tenon diameters....
So, you can switch 'em out...
(For one who appreciates irony...this can be entertaining...because you get folks with neckless 10M's needing a neck for it...so they either buy an original, bona fide 10M neck which goes for around $500 on eFlay and are hella few and far between...or they have one made...which goes for around...um....$500.
While they could pick up a Mexico 16M project horn for about $150....keep the neck, and turn and sell the 16M body for $50).