John Hall explained to me that RIC instruments are painted in batches of 25. During the ‘60s and ‘70s, the letters A through Y were marked in the cavity alongside the model number so the painters could tell the order in which the instruments in the batch were painted so that each would receive the proper drying time before the next coat of paint or clearcoat. That "U" is simply a factory code for the painting sequence. It has nothing to do with the dating or other features of the instrument.
I agree with your comment about OS and WB 360’s because of the appointments.
I do want to point out that the ‘74 360/12 pictured on right does have OS written in the cavity.
I have a ‘65 365 OS Natural. Noted as 365 U in the control cavity. “U”?
John Hall explained to me that RIC instruments are painted in batches of 25. During the ‘60s and ‘70s, the letters A through Y were marked in the cavity alongside the model number so the painters could tell the order in which the instruments in the batch were painted so that each would receive the proper drying time before the next coat of paint or clearcoat. That "U" is simply a factory code for the painting sequence. It has nothing to do with the dating or other features of the instrument.
That’s a great detail. Thanks, Ron!