You'll find an explanation of the 4 types in the link that Dal provided. In summary, different variations of flat springs for the top-lever and ejectors were used in Types 1, 2 & 3, whereas coil springs were used in the Type 4. All 4 types used flat mainsprings.
I've heard it said that the Type 4 with coil springs is the best (heard it from a guy selling a Type 4). When I looked into getting a spare set of flat springs for my gun, I learned they're tougher to get for a 20 ga. than they are for a 12 ga. I was also told by one source, "Call me when you actually break a spring. I'll probably never hear from you."
Those who have access to back-issues of Clay Shooting USA may be able to locate an article called: "Miroku - The Best Gun Maker You Never Heard Of".
Last edited by ninepointer; 02/21/17 07:23 PM.