Hmmmm. You might be on to something. I assumed with the Ishy screw and not seeing any stamps on the receiver flat that it had been scrubbed but you piqued my curiosity and with a magnifying glass and angled light source I can just make out o4 MkI (F) FT so I assume the N at the beginning and the R at the end are illegible. So maybe no Ishapore scrubbing, just a worn lightly struck stamp. Further, the receiver looks like it was painted with Suncorite further hiding the stamps.
Do you think a 4 digit serial number Shirley is worthy of preservation as is?