RAF Royal Observer Corps field telephone enquiry

I have a pair of ROC I’m trying to find out more about them the label on the inside is marked

telephone portable for observer post, RAF ref no.10g/125

Issue pre 1942 due to the letter abreviation chart A= ac B= beer etc

The is a a brass tag: Telephone observer DIAG AD 163 B and hand painted on the back FH 38 which i was wondering could that be a unit or area marking