Thread drift is OK here as it helps to show the differentiation between the two 360s.
400/360 is much too big and powerful for a round block Webley.
There is a picture of the chamber cast showing it to be the 2-1/4" cartridge.
The original 360 2-1/4" BPE used a charge of 50grs and my suspicion is that this particular rifle could have been proved for the later NfB loading but it does have the BPE load stamped on it.