You will need to measure the drop, from your RB's extended line-of-sight, down to the tang, and go shopping at gun shows with the measurement in hand.
That's how I found($50) & fitted a ladder-type vernier tang sight with a short staff to my #4 roller a few years ago.
And, FWIW, Lyman & Marble's could also most likely help you out, once you supply them with said measurement - as pretty much any of their short bases would fit the tang, only needing a staff of the proper height & angle/stop (which can be user adjustable).
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