I saw a propane F40 for sale on craigslist. Did the factory produce them or are they all retrofits?
For years the “Official” answer has been, Ferguson (MHF) never built a production version of a propane or LP gas tractor. Prototypes of a Ferguson 45, the proto-type propane or LP gas F-40, were built, and at least one example has been found. Development of that proto-type led to the production of the Massey Ferguson 50 propane or LP gas tractor but never resulted in a production F-40 propane or LP tractor.
That having been said, a number of propane of LP gas Ferguson 40 tractors have turned up in the South and the Southwest where propane or LP gas was a popular fuel for tractors. These tractors are believed to be conversions however, the quality of the conversion is so good it looks like it was installed at the factory.
There have been articles in Furrows about at least one of these tractors, and a lot of discussion over the years whether these tractors were built by the company that could have built the MF-50 propane or LP gas tractor. FENA member Mickey Keener in Texas has one of these tractors, and has additional information on this subject. From what I have heard and read, these F-40 conversions are very nice and do not appear to be one of the crude conversions farmers or owners sometimes did. If the conversion you are looking at is one of these professional conversions I believe it would be a very nice tractor to restore and a very collectable variant.