I suppose that technically it is an experimental as it is not a regular sealed design, but that round may have seen limited issue to Special Forces. I don't know how many were made, but probably a typical lot of perhaps 100,000?
It has a 150 grain flat based bullet as shown in the centre of theattached picture. The left hand bullet is a normal 115 grain Mark 2z and the right hand bullet a 170 grain boat-tail from a 1956 dated RG round. There have been a number of different British flat and boat-tailed sub sonic loadings since the end of WW2 weighing 150, 160, 165 and 170 grains. Some in 1945 dated Blackpole cases were almost certainly loaded in the 1948-49 period.
They are usually identified by a coloured spot of paint on the base of the case, but some have a special headstamp like yours.
Sorry about the quality of the picture but it was a quick composite I put together for this post.
The purple paint, actually officially called "violet", is used on all types of munitions to indicate the experimental nature of the store, not just on small arms ammunition.
Regards
TonyE