hsp
Ian
As you surmised "hsp" means Hamilton Standard Propeller and there were many licensed manufacturers including de Havilland themselves. As for the Merlin, well Ford too was a large scale builder who did much to improve the breed.
Regarding nomenclature, a number of incorrect notations or spellings find their way into records such as MkXXV which then jars with the official history.
Hamilton Standard was then and still is regarded highly for the hydromatic propeller which uses high pressure oil to move a piston fore and aft, this motion being translated through cams into a change of prop blade angle with a governor controlling speed. But like all inventions and all training there is an outside chance that something quite unplanned goes wrong along the way. There is always an enquiry to try and ensure that it can't happen again but as pilots will tell you, sometimes there is no way out.