See here:

http://www.gunproof.com/Proof_Memoranda/RULESOFP.PDF

Page 7:

"6.If a barrel of one sort is converted into a barrel of another sort, (for example a short barrel into a longer barrel or a barrel into a sound moderated barrel or from a fixed choke to an interchangeable choke,) the barrel shall be deemed out of proof from the time when the conversion is begun. (See sections 115 and 130 of the Principal Act, as amended.)"

The reproof mark is a crown over CR (bottom of page 16).

Regards - Ian Forrester