In Japan, MUSCLEs were known as Kinkeshi.  But not all Kinkeshi sculpts were imported into the MUSCLE line!  The Japanese Kinnikuman line had over 400 figures available, while we here in America only had 236. That means at least 164 more Kinnikuman exist! Also, Kinkeshi were cast in different plastic colors as MUSCLEs.  Shown below are some of these figures. Each of these Kinnikuman figures have the usual Kinnikuman / MUSCLE trademark "Y/S*N*T".

Non-MUSCLE sculpt Kinkeshi

MUSCLE sculpt Kinkeshi

MUSCLEs vs Kinkeshi

There's a definite difference in the Kinnikuman plastic compared with the MUSCLE plastic. The Kinnikuman plastic is softer and more flexible. However, the MUSCLE plastic seems to catch the details of the sculpts better. Check out the scan below of a Kinnikuman and MUSCLE Hand for details.