Just an update so other folk will know for the future. Cary says it is electrically the same but will not fit because of the size.
Also, they don't recommend a directly heated rectifier 5R4 or 5U4 on the SLP-05. " Use
indirectly heated tubes like the 5AR4, 5V4/GZ34, CV1377,
GZ32, GZ33, CV729."
In any case, the reply came too late as the seller couldn't wait and sold the Brimar. Not that it mattered in the end.