gas cap questions galore..

[jocko]

Let me tell you what I THINK I know, but since I'm also thinkin I might be wrong, I'm hoping someone can square me away...

1/2 ton:
67-71 caps were unvented (or, actually, what I mean is that they were for "unvented" fuel tanks in the cab)
72 caps were vented

3/4 ton:
67-72 caps vented

a) were the caps for different dia opening filler necks as well? (I seem to have 1.5" and 2" floating around in my head) - and I think the vented one was the larger dia??

b) So, will a 71 1/2 ton cap fit on a 72 3/4 ton truck?

c) Is there a way to tell if a cap is for a vented tank (evap emission 72 thing) just by looking at the cap only (or vice versa, a way to tell a non-vented cap just by looking at the cap only).

d) I believe all aux tank setups, factory NL2 and dealer-installed dual aux tanks) were all unvented. So, which cap would a 72 with an NL2 aux tank use on the aux tank? A 67-71 non-vented cap? Or what id the truck is a 3/4 tonner? A 67-72 3/4 ton cap? I THINK the aux tanks all had the smaller dia filler neck (and associated cap to go with it..?), all years.

Appreciate your help.