All fuel inlet elbows I've seen look like the one on top. Maybe I haven't paid much attention but never saw one like the skinnier one.
The one on the bottom looks like it has 1/8" pipe thread at both ends...
I think Keith is right. Looks to me like the same thread on both ends. I'm not sure it's pipe thread though. It doesn't look tapered.
The bottom one looks like a typical later type ell made for brass compression rings and an appropriate "nut" at both ends - if the openings have a rebate that fits standard tubing closely, that's probably what it is. It's not "original" Model T - the top one is.
Here are some close ups.
That looks like 1/8" pipe thread on one end. Could be used on a T carb, but the original police will spot it and dock points.
How about at the other end of a fuel line??, 26/7
I was looking at some fuel 90s I have and the ends are slightly different.