Dudley - I checked out 12R5 and am not warm and fuzzy about the comparison of markings. I think you are right-on about the B relief and the 2nd row right pane have several good possibilities.
12R5 - No plate mark (PM) left of ornament C.
No PM's in top right scroll.
No horizontal line below ornament D.
Does have a fain PM near bottom of right plume.
12R5 has no PM in T of CENT.
19R5 - Has PM in T of CENT.
Does have the break in the top line above the S in US.
Missing horizontal line through the N of ONE.
17 thru 19R5 have a PM in the T of CENT. My 'better' scan soes show a faint mark there.
Though Siegel has an example of a 15R and Worthpoint an 18R, neither are clear enough to help here.
Maybe these scans will help (make at 2400 dpi):