The Broad arrow stamp on the barrel would suggest it is a military conversion.
I have a 1918 .22 SMLE by Parker Hale that does not have the No2MkVI stamp; but the barrel also has the broad Arrow and dated 18. It appears to be a solid .22 barrel, not tubed.
(Unless the join is invisible). Great fun to shoot and surprisingly accurate.

Do you have a visible date stamp on the barrel?