I have a pre-27. On the frame under the crane assembly where later on the mode number would be stamped, it is marked XX 9. There is another number matching the crane/yoke that I assume is the assembly number. Don’t know what the XX 9 means?
Most likely they're either fitter's or inspector's marks - or both. Those kinds of markings were added to guns at various points as they travelled from one operation to the next so they could identify the people who had either performed specific operations - or inspected them.
Marking question
Linear Mode
