[Denis]

There are actually 2 different dates. The first is the date the housing was cast. This is a julian date (day only) just underneath the '56791142' casting number. Yours looks to be a '139' which is the 139th day of the year.

The second date is stamped on a machined pad where the cover bolts on.This is the date of assembly, and is always later than the cast date on the box. The format for this second date is as Mike described in his posting (i.e. A or B, julian day, year digit). I can't make yours out from the picture.