Mexico Diez Centavos A18 Identify Help Please

Hello All

#1. This stamp has a watermark which I can not identify.
The watermark resembles a square block style of print. ( see picture )
Is this the style of print used for the " C " in wmk #152 ?
Your opinions please.

#2. It has a right angle mark on the back. ( see scan ) What might this be?
This stamp has no watermark. And I'm not sure about the color.
What scott number might this be ?

Thank-you.

#2 The angle thing on the back is part of a printed rectangle from an envelope or post card, in English often with "place stamp here" or somesuch. The traces of yellowish paper is from that envelope, too. Seems stupid now, but putting stamps in the upper right corner wasn't a convention back then.