BudsGunShop.com - I just baught this rifle,

In 1953 Marlin Firearms was issued U.S. Patent 3,100,358 for what was named MicroGroove Rifling, which was a departure from the standard "Ballard," or cut rifling. One purpose of Microgroove rifling was to increase the speed of producing rifle barrels. Microgroove rifling is described in the patent as having 5 grooves for every 1/10 of an inch bore diameter, and that the driving side of each land would be "tangentially disposed" to prevent accumulating fouling in use.
Marlin introduced Microgroove rifling in their .22 rimfire barrels in July 1953, with 16 grooves that were .014" wide, and nominally .0015" deep.

I is micro groove rifling. It’s like a spline instead of six lands and grooves their are a lot more. I was not aware they brought that back. They stopped doing it a long time ago and went with traditional rifling. I have a 1978 marlin with micro groove rifling