Great collection. To follow some earlier discussion, here are the uncommon 51-V along with a relatively uncommon variant of the little ringtop 52 1/2 V, in this case with gold tone crown the 01952 1/2V. Turns out the 51-V is about same size, overall, as 52 1/2V. The nib is smaller and the section tapers in more, and on this example the metal trimmed 52 1/2V cap is tad longer, but overall length about same and threads are compatible: same diameter. Note the really tiny nib on the 51-V. The larger nib in the metal ring top is just a #2 size.
regards
David