The dive shop up in Moorhead had a set of Twin 50s made from CO2 fire extinguisher tanks that we guessed were 50s. They had dimple feet bottoms, were 1800 psi, and stood a little taller that 38s. On the other hand, I have those surplus "Twin 48s" that are quite a bit taller than 38s.
(PS - I got the "48" number from an old Scott Lung)
Another thought is that they may be something between 38 and 50 cf. I have an oxygen tank that is identical to a 38 but is about 2/3 as long. Diver Don actually has a set of twins in the 2/3 length. So maybe there's an in-between size in the other direction, say... 45 cf?
OR,... 1/4 inch difference in length is insignificant and you have Twin 38s!