Hi Peter - Thanks for the response. I have a Stampworld membership but never think to use it for the great cataloging. I did try to determine it in my Scott catalog at #s 174 - 241 but the edges for the perfs look more like a rouletted stamp than perforated. I could not see any way to measure them.
Looks like Mexico Scott No. 188 to me, cat. value $18 if it's back is OK. Perforation in this issue was of very poor quality, with many stamps having no perforation on one or two sides (separated by scissors). I've had (and still have) some of these 20c carmine stamps; they come in many shades, heights and widths.
I don't think it's a rouletted stamp. It's just a haphazard perforation (perhaps pinhole perforation).