Between 1962 and 1968 Marlin used a two letter date code stamped on the barrel, it gives a date when the barrel was stamped and a pretty good indication when the rifle was assembled.Since no serial number, all you can know is that it was made before the 1968 law requiring serial numbers. I think the 99m1 was first manufactured in 1964. It is the same action as the model 60. Some bolt hold-open devices had a spring loaded button on top of the bolt lever, others required that you pull the lever back fully, then pull lever outward to lock it open. Pushing it in released the bolt to return to closed position.