Anderson found 'old mark' stamps being used on forgeries in several Mexican shops in the 1930s, photographed impressions from those found are shown in Boylan's book - they include variants of the marks on this piece, though none identical. Perhaps he might have been suspicious in particular of the upturned wings and general appearance of the 'eagle' in this set of marks, which is atypical of the Mexican eagle marks, having instead, to my eye, a rather distinctive, sort of 'Imperial eagle' look...

~Cheryl