Doug,
I e-mailed the good folks at Dwyer, and got a prompt reply. I also called Rig-Right and, contrary to the bad rap they have been getting on this board, I got a live person on the phone. . .
Ok, so it was someone's teenage son, but he took my information down and said someone would call me back...
A few hours later, I got the call back from someone saying that they "normally stock that item, but they're out of them right now", A new batch would be in next week, and its only $289 for the whole setup, including bolts and an instruction sheet!
Wow, what a deal... Almost $300 at Rip Right for a part that I can get for $80 elsewhere!... Hmmm, Tempting

I think not!
I'll be ordering from Dwyer tomorrow.
Edit: Friday Morning 10:15
Jay Kirby in customer support at Dwyer Masts was a big help. He directed me to their online ordering, and advised me as to what I needed.
Although their masts are a slightly different shape than the Schaefer mast I have, the solution was to keep the 'step' that bolts to my mast, and have it re-drilled to the bolt pattern of the hinged plate on their tabernacle. I think that the nicest feature that their tabernacle has is that it hinges either way, forward or aft, by simply removing one pin or the other. This will make it easier to negotiate the mast down whether on the trailer or in the water.
Cost: under $80 including shipping, which saved me some
serious money to spend on beer for the
bottom painting party planned for next weekend!