Laurie, even though the right-side ornaments are partially cut away you can see that they are more complete than they would be on a #24. Also, the break in the top frame line is much smaller than is the case on #24. This is a Plate 4 C Relief, which as sinclair said produced both #21 and #22.
Thanks Dudley, I missed out on this stamp unfortunately.
With your assistance and also from the other posters/Oracles on this thread, I'm slowly learning what to look for.