The answer to your question is YES and NO, the action, ejectors, locking mecahnism and top lever are almost the same albeit that the overall length of the action is smaller and the first 3 interations utilised leaf springs, the 4th was an all coil spring arrangement and ceased production in 1976. The big difference is in the hammer and sear system., the under barrel hammer pivots off the top of the action and the over barrell pivots off the bottom of the action. The firing pins use a dual pin system with the primer striker being removable through the face of the standing breech, the hammers have a lug out the side which strikes the pin which travells in a direct straicht line 90 degrees to the standing breech unlike most other over and unders which are offset.
If you go to this site
http://www.wisnersinc.com/main.htm click on shotguns, click on Miroku/Mossberg, Click on CLICK HERE for Miroku Firearm Model and Spring info, keep scrolling down and you will learn all there is ti know about these fine old guns.
Hotrack