What brand of pistons do you have? The early Hepolite Powermax pistons for Commandos had the shallow oil ring set. I don't know if the standard Hepolite pistons for the early Commando also have used them. I'm not sure when they changed to the deeper rings. My '71 PR came with the Powermax pistons with the shallow oil rings. It used to be easy to get the shallow ring sets, and might still be, if you have the right part no. They were a bit more likely to get out of the groove while installing the cylinder, so you had to be careful with them. I had a base for holding pistons in the lathe, so I chucked one set of Powermax pistons up and cut the oil ring groove deeper to match the later rings. That's not a practice anyone recommends now, but it worked ok, and the pistons worked fine in the race bike.
Ken