Parkerised No5 MkI

Hi everybody,
I found this little No5 MkI. It has all matching numbers also on the forend.
The only thing that makes me thinking is that the surface of the metal parts is parkerised. I have never seen this before. Can anybody outthere in forumland give some advice about this finish, is it FTR. I have not found any other markings than british, but what means the B and the circled D? To buy or not to buy that is the questione.

Attachment 27324Attachment 27325Attachment 27326Attachment 27327Attachment 27328

The D within a circle indicates a later production barrel where instead of indexing the barrel thread to the other important features of the barrel, such as the knox form, extractor way and bayonet lugs etc, the breeching up thread was machine FIRST onto the partially machined and rifled barrel. Only THEN were the other indexed parts finally machined. It saved a LOT of almost fully machined barrels going to waste

There are other opinions, such as that it was draw-lapped or ball burnished and such like - even to that it was one selected for its accuracy - but none of those others ever made it into the Armourers note books. But I could be wrong..................

Thanks Peter for the info.
Was that type of finish for the metal parts ever used on No5 MkI? I have seen this only on Australian SMLE FTR and Canadian No4 FTR rifles. Is there any idea?

The No.5Mk.1 had Suncorite 259 paint baked on over Parkerizing. Yours may only have the paint removed or been Parkerized again at some point. Nice rifle.