A quick run through the gallery on the T206 Resource (
http://t206resource.com/Plank-Gallery.html) shows:
Sweet Caporal 150: 12
Sweet Caporal 350: 34
Piedmont 150: 4
Back Unknown: 10
Total: 60
I went quickly so this isn't perfect, and I didn't bother to try to guess whether the scans without backs are 150 or 350 (though it wouldn't be too hard for the decent scans given the significant difference in image quality). But there are a lot more 350s than 150s.