I just serviced two Sahara 750FH and two Symetre 750FL reels. The rings that Shimano sent me were way too thick for this series of reels (part# RD8147). The Shimano parts guy told me that the old number was crossed to this new number. Not! Now I have to complain in hopes of returning to get a full refund.
Here's what I used for replacement rings: bicycle innertube 26" x 1 1/4" x 1 3/8" cut into 1/8" wide straight strips with scissors. They are stiff enough to cover the gap leading to the gearbox. Fit perfect!