Just in case. . . Your Tripehound already has markings for Maynard’s N534 (only 1 digit off N533). Purchase HGW 232020 (available on ebay) which has markings for Collishaw’s Black Maria Camel B6390 (from which take the trailing “3” for the serial), a little surgery on the “C”, an’ Bob’s yer Uncle! Or, in this case Ray.
HTH, Rob