Measured at the back, using the standard follower, they run from 4+7/8” to 5”.
Since the spring is shortened to 13 coils, you can cut the mag to 4+3/4” and shorten the standard follower ¼”.
I don’t have a “super rare” shortened one made from an Indian converted 19 round mag. The followers are modified in the Indian mags. You might have to add a little length to compensate or take more off the bottom of the follower.
I wonder if he has a “ultra rare” 7.62x25mm Tokarev Sten mag.