Fascinating. I am not an expert but you got me interested. It looks like there is a lot of confusion among the
ebay listings, and even among presumed experts.
This is my understanding of the two types, II and IIa, with curl filled in green or red:


Seems pretty clear the first image has the bottom curl filled in green and the second image has the curl "open" and filled in red.
However the second photo is from PF Certificate 596993 that was submitted as WX12a (in a block) but PF decided that it was not WX12a but "rather all examples are WX12".
Furthermore, the top photo is from the web site of Christmas Seal dealer William J Kozersky where he has photos of what looks like (to me) both type II and IIa listed as type II WX12.


I am confused. Hopefully an expert here or someone with Greene's catalog can chime in and straighten this out.
Based on this, yours looks like WX12a, unless I have interpreted the types backwards. Too bad it is not tied.