Moyer Marine makes a replacement back plate for most Oberdorfer pumps on A4s that employs an o-ring and four captured knurled-head screws. The advantages are three-fold: No more having to replace the gasket, no more having to get a screw-driver on to the screws--possibly from an awkward angle/position, and no more dropping screws, etc. into the bilge
Perhaps the Speed Seal cover offers some of the same advantages?
Thanks for the picture of the new impeller, Hal!