Hi Meg,
Sorry for the late reply.
Looking at the original it looks like she does join up as a Wren Steward. I have trawled all of my sources and I cannot find a specific reference to HGR so I am guessing here that it means higher [unspecialised] rate and of course happy to be corrected.
Wrens joining on general duties or unspecialised would go on to an unspecialised rate and would progress depending on successful completion of training and recommendation.
It would appear she changes categories to communications and specialises in teleprinter operations. Normally, on completion of training, she would be put on the lower specialised rate of pay and after three months and a recommendation she would move to the higher specialised rate of pay.
Regards
Hugh