Yes, it seems a similar idea. The Rotax heavy maintenance manual says there is a shim that should only be fitted if the desired oil pressure is not obtained without it.
Looking at the diagrams, it seems the pressrue relief system is just a ball on top of a spring, with a plug screw. The shim shown looks just like a thick washer, as you suggest, which presumably just acts to compress the spring a little more.
Looks like I may be getting my hands dirty at the weekend
