I am uncertain whether the metal Conns also followed the conventional serial number progression, but if so, it puts yours at 1913. That is right on the cusp for HP/LP instruments in the U.S., and it would be expected just as likely to be the former.
That said, you did not mention a HP stamp. Conn DID briefly stamp LP, but I am not sure the reverse is true.
So, a C, you say? Would you mind assembling it, and providing me with the measurement from the top of the barrel to the end of the bell?
Thanks for sharing that.