Judging by these 1970 catalog scans indicating that the 4x12 cabs were 15" deep, I assume these 12" deep old OM cabs must be from the late 1960s? There was a mint OM stack on some collectors site (one of the pictures below is from that) and it was dated 1968 but I don't know how accurate that is.
There is another thread I just found which was posted a year ago where a member also has a 12" deep OM cab, again identical to mine, and was trying to date it. Is it possible that these were only made in 1968 / 1969? Does anyone have a catalog?

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These pics are not mine, but my cab looks exactly the same as these (aside from replacement plastic handles drilled out when it was butchered in the 1980s


