Bruce,
I asked one who worked most of his life on these at Parker Hale.......Bill Smallwood and here is his answer in lockdown:
A photo of the face of the barrel would be good as the only barrels for the T4 that I saw were angled all round the face, so as to do away with an extractor groove, and also this did away with the need to index the barrels.
Early barrels could have been made with a flat face and extractor groove filed in when the barrel was fitted and headspaced, but this would be a very lengthy operation and may be the reason they went to the all round angled barrel.
Also I have no doubt longer barrels were made maybe in small batches to aid accuracy etc etc.
Also the fact it has been roll marked with "Parker Hale" shows that it must have been fitted to a rifle as barrels were never marked until fitted so the name would be on the top of the barrel.
A photo would also show the type of shoulder it has.
Also be of interest to find out where it was acquired.