This is the result of using the oil from the 60’s and for some years later. Now that you have it running, today’s oil, along with the expansion and contraction of the pistons and rings (why you need an end gap) the rings will free up over time. I suspect you will continue to see an increase in compression if you continue to run it. Some may still be stuck. Others may be partially stuck. It can take some time to get them all completely free. Tubs
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.