You lost me. If the gun was successfully converted, why would it not handle the commercial super rounds? Or was the conversion designed to shoot wadcutters, the way the Smith m.52 was designed for .38 Special wadcutters only. I would think your dad's original article would have the length detail.
A reference available to you is the SAAMI site
. Click on Cartridge and Chamber drawings, then on Pistol and Revolver, then, from the list, on the cartridge in question. The 9 mm Luger
has a maximum cartridge overall length (COL) of 1.169". The .38 Super
has 1.280" maximum COL. That's a difference of 0.111". Not a huge number. Many magazines will handle a little more length than the normal SAAMI maximum, and 0.111" is not an impossible number. Plus, if you buy a hollow point or almost any bullet shape that's not a round nose, it will have less than maximum COL in the super. Hornady shows 1.210" being a common length for a number of their bullets in this round, and only 0.041" longer than the 9 mm Luger maximum, so they may fit. You'd just have to try it and see.