Thanks for the quick reply.
I went back to the local Penta service centre and they advised that the 3840525 was the original spec and the 3581621 is the new spec, which is a larger capacity filter. Not sure if that is the case, of if it was just what he thought would be a reasonable response as to why my engine had a Volvo 3581621 filter on it.
In any case, I have an engine with fresh oil now and a 1 year old filter. I used the motor for maybe 40 hours...what are thoughts on if that filter will hold for another season, or if I should do another oil change in the spring, when we relaunch our boats?