Thank you for replying, both

The page it's on is the one listing next of kin and home and abroad service as well as marriages and birth of children.
The line reads:
"Appointed guardian Mrs E Thompson (17/21st PTO 105/36)"
There was nothing on the next page that might suggest PTO meant that...
A guardian would have been appointed as my grandmother died 22/9/1936 so I would think the guardian (her mother) would have been appointed shortly before her death.
Maybe it is a reference to some other document I don't have.
Oh well

thanks so much for looking at it
Emma-Jane