Odd stock on a Mod D.

I’m a sucker for ugly orphans and underdogs. The kind of person who chooses the runt of the litter in a pet shop. I understand the appeal of “minty” air guns but my heart and wallet are drawn towards the “been there, done that and my latest owner no longer wants me” guns.
Over the past weekend I attended a two day auction of 1,800 lots. Most were firearms and there was a less than usual smattering of air rifles. Even I chose not to bid on a very beat up Airsporter MKI that went for $30. But there was one rifle that tugged at my heart.
It was a .177 Improved Model D. The original finish was mostly gone to be replaced by a motley brown patina and bare metal. All the parts were there and they looked original. When I cocked it there was some reassuring resistance from a firm spring or springs but when I let the piston return with the tap up there was no indication of any resistance from compressed air. The stock had three holes drilled in it at the wrist and a half inch square cut from the left hand side. At some time in the past I assume this was done to accommodate a rear sight. At the bottom of the wrist extending back a skilful past owner had grafted on to the straight stock a half pistol grip. This has been very well done and appears to be a compromise between the straight stock and the pistol grip stock offered by BSA. Takes a while for the eye to get used to it but it feels comfortable to hold.
I deduce that a past owner was an air rifle target shooter who was able to customise or have customised his standard model for target shooting. As English style air rifle target shooting did not catch on in New Zealand it is likely that this rifle was brought out here by an immigrant many years ago. (I myself brought out a much loved “Improved” when I immigrated 40 years ago.) Over the years the sights have been removed and the rifle has been neglected and badly stored.
All in all this was one puppy I had to rescue. Only one other person bid but I won it for £60. As I type 20862 is on the sofa beside me. I doubt that it will ever regain its former glory but I’ll clean it up and take care of it and give it the respect that another long gone owner once gave it.
I’ll ask Garvin to post some pictures on my behalf. Any ideas as to when 20862 left the factory?
Barry