@Carl N. Brown Thank you for posting that video. It is definitely full of great information.
After looking at my 1941 Lithgow that has been sporterized, I can see where the No 6 Mk I/1 influenced how people sporterized the No 1 Mk 3 rifles. Here is a photo of my 1941 No 1 Mk 3 * Lithgow.
As far as what the OP has, I would love to see a close up of all marking. That would help us identify what he has.
After looking at my 1941 Lithgow that has been sporterized, I can see where the No 6 Mk I/1 influenced how people sporterized the No 1 Mk 3 rifles. Here is a photo of my 1941 No 1 Mk 3 * Lithgow.
As far as what the OP has, I would love to see a close up of all marking. That would help us identify what he has.