Many thanks for the help, judging by the pictures on the link they could be the standard type of tampions used when at sea, the crested versions were fitted in harbour probaby during important visits. In my Prince George build thread I found out that after 1917 they were all changed to one standard type of tampion. I'm beginning to suspect that the crest was "G.R." -The then Prince George's initials before he became King George V.
I've contacted the RN archive library at Portsmouth but the automatic reply said that it could take up to a month for a proper response.