Starting in '66, the engineering numbers were a soldered tag instead of stamped into the top tank. Only 'W-MO' and a date stamp was there. ...............
Just to add the change took place during July of 65 at the Modine plant
A couple of examples from that month from a posting of 66 running changes over on CMF
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This example dated 7 65 does have the raised stamping showing the application code. C4ZE G2. Found a fair number of these on very late 65's in my records.
This example dated 7 65 does not have the raised stamp showing the application like the earlier version above.