Dodge has made some strange specialty parts through the years. in the mid 60s, that made a slant 6 727 (and a matching manual trans bell) that also had a LA small block bolt pattern. Both were for heavy duty truck applications with the slant 6 and quite rare.

I suspect the mentioned bell for the flathead 6/ V8 is a pretty rare critter as well.
Rare in Mopar speak usually represents many $$$$$$$.
Gene