I suggest, when shows start again, checking the 748 FDCs generally or looking for something like these with either 21590 LR Pl Blk or a LR 100 plate single.
As to the Plate number FDCs, those can be requested as a "want list" order from the usual FDC dealers. Likewise the request can make a simple classified or want ad in the American First Day Cover Society's, APS's or United States Stamp Society's respective journals. This way you get what you want without driving collectors or dealers insane looking for the missing period itself. Plate numbers stand out.
I am suggesting this under the impression that as both higher and lower plate numbers were available on the FD, yours should be too.
What you really want to find is an FD or other date cover with an OB/GYN corner card. Could be a start to a topical collection.