From the original machining I'd guess it was for a larger bed but less swing Monarch - simply because the original base is "wider" (center of vee to center of flat way) than the adapting piece.
See what center height is without adapter - my 16 / 18 1/2" actual CW16 is exactly 9" to center line of spindle from flat way
I was sort of assuming that Monarch made the adapter block so as not to have a gazillion different steady rest castings for their amazingly varied lathe models. OTOH, now that you mention it.... if it was Monarch standard issue there probably would be some paint on the block...so perhaps it was a shop adaption sure enough...fits the 612 perfectly though...
If with the adapter off it measures 9" tall to center from flat way and the flat way center to vee way center is 8 1/2", it likely was made for the CW / CY 16" lathes
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