You can easily tell 100 round from 75 round if you can't see the size difference by the number of scroll circles. The 100 has 4, the 75 has 3.
I do believe that the Chinese were the only ones to make a 100 round drum. The 75 rounder came in 2 styles, the Chinese and the European. There were some Chinese style mags to allegedly come from Europe.
It you decide to load them, be extremely careful releasing the spring. Push the release button with a long screwdriver or punch. There is a nice finger/hand shredder under the release that can and will do a number on your hand if not very careful. Also be ready for a loud band as the spring releases which is normal.