Welcome aboard KP31,
In my collection, I have an early Savage Model of 1928, 18344, which is in the UK and the trigger frame has the serial number ground off that too. It was converted to semi auto, back in the day when were allowed that privilege in the UK, before being deactivated in 1990.
18344 lower blank SN.JPG 151.5K
I also have a later Savage 1928, 85140, again deactivated, and that has a force match for the trigger frame and receiver, although the original serial number remains and has 85140 added below it.
85140 Trigger assembly SN.JPG 161.5K
In the past I have read that the mismatched Thompson serial numbers were not unusual in British army service, as it was easier to replace the frame than to change barrels, when the weapon was in for repairs with the REME field armourers.