You might find that you can replace the block with similar, using the same base plate. For example, if the Shaefer is a 57mm sheave, you might be able to remove the bock from the plate, and replace with a 57mm Harken. On my smaller boat, I was able to do this with my Harken swivel jammer for the main sheet, and replaced the block with a ratcheting block. It was just a clevis pin that held the block on the base, and when I removed the small shackle that came with the ratchet block, it fit right down into the jammer. Could be the same for your standup blocks.
Brian