The UD42 magazine is different from all other 9MM magazines because when it was designed to supplement the U.S. military Thompson submachine gun, the designer wisely chose to use the same magazine as the TSMG used (although that may have been required by the military). The original design was provided for testing in 45 ACP caliber, using the same magazines, which were abundantly available and totally reliable. The ammunition supply chain would not require anything new.
When the .45 weapon was not selected for U.S. use, alternate users were in need of 9MM weapons. The design and tooling was already established for use of that style magazine (utilizing a "T"-spine for mounting to the weapon). Converting to 9mm would only require some small changes to the weapon if they left the same design for mounting the magazine. Manufacturing them in 9mm was also not a problem for the many manufacturers of the TSMG magazines.
If a UD42 and a TSMG magazine are placed side by side, "T"-spine up, one sees that the mounting is absolutely identical.
I am currently experimenting with creating an alternate magazine when OEM UD42 mags are not available, or maybe more than just 20 rounds are desired. The combining of a 30 round TSMG mag outer housing and a UZI 32 round mag (one I had at hand to compare) pushed up inside will do the trick. Another that may also work is a ASC Colt style made for AR15s in 9mm. 20 or 32 round versions are available. The four insertion limit protrusions on the UZI mag need removal (not so with the ASC mags). The top of the TSMG 45 mag has to have some of the feed lips removed to clear the access into the UD42 receiver in order to latch up. A later post will provide pictures and dimensions. A little more off the bottom to match the lengths and allow access to the base plate of the UZI mag. If done judiciously, the finished project, when mounted to the gun, should be almost indistinguishable from an OEM mag, other than a little longer and a touch fatter. A 20 round ASC mag coupled with a 20 round TSMG mag housing would look a lot closer to a UD42 OEM mag. Only experts will know the difference.