Clarification/Follow-up by voiceguy2000 on 04/23/05 5:02 pm:
The original Pocket Companion was published in 1935. The Royal Society of Teachers was formed in 1929, wound up by court order in the late 1940s. There does not seem to be much information in cyberspace about Arthur Mercer, but if he was a proud member of this newly formed society, it seems plausible that he might have mentioned this as a credential.
Clarification/Follow-up by denberg on 04/23/05 10:42 pm:
Thanks voiceguy, that's an excellent answer - I certainly hadn't heard of that, and it seems very likely.