I have some theories: The O and U on the tang was a reminder; recall, JMB wanted this to be the first-ever "every-man's" O/U, so that the Superposed would be the first O/U most men ever had. I think it occurred on most all guns whether it was a DT, double-single, or just single, because the tangs were all stamped that way regardless of the trigger option.
I think "C" means Custom ordered, as was my gun. Thus the extra engraving, etc. Once production processes improved, this was eliminated as a tracking device. Just a guess. Not sure about a "P". I too think the gold S was deleted for cost cutting measures; Browning only sold 201 guns in 1934, then dropped the price 33% and sales took off.
The FN on the opening lever may have been made for FN's European sold guns, and thus placed on the Custom ordered guns for an added touch, or they may have just been the original batch of levers, and whatever was in the box at the time got installed on the gun. This "FN" may too have been eliminated as a cost savings in 1934-35.
