The answer
Dear all,
Many thanks for your very helpful suggestions, and apologies for the delay in providing the denouement. 146fixer more or less hit the nail on the head. On mechanical inspection, it transpired that:-
1. The prop was NOT in fact one approved for use on a Cessna 150, but rather was intended for the Bolkow 208 Junior only. As if that were not enough...
2. One blade was an eighth of an inch shorter than the other, and both blades were out of alignment by as much as 4 degrees from one station to the next. On top of that...
3. Both spinner back plates were cracked, and had to be junked.
The cost of fixing all this? Around $5,000, including labour. None of it picked up by the expensive (and well-regarded) independent maintenance shop that I brought in from a neighbouring airfield to conduct a pre-purchase inspection. Nor is there any indication in the aircraft log-books as to why the non-approved prop was put on the aircraft in the first place (I wonder, I wonder...just why does one replace a prop??), or why its grotesquely non-airworthy condition was not noticed by the specialist outfit that, according to those same log-books, inspected and rebalanced it less than a year before I became the owner of the aircraft to which it was attached.
This matter is now the subject of an investigation by the proper authorities.