This has nothing to do with a Vincent in a barn but years ago I was working at a computer company and a vendor, who had a scrap computer business, was moving his warehouse. He knew I liked bikes and he asked me if i wanted to buy an old BSA Gold Star. I figured, if it was a BSA "Gold Star", it was probably one of the unit 500 jobs that used the same name but were nothing like the original. At the time I was hanging out and racing with Jerry. Jerry had something that I never did, money. So I called my buddy, gave him the address and told him to go have a look. I wrote the whole thing off as just another missed bike. In less than an hour I get a call from Jerry. "Holly crap" he says. "It is a for real BSA Gold Star. It is laying on its side under a bunch of computer junk and I can see the right side of the engine."
After many frantic visits to various ATMs, he gave the guy $500 and took possession. He couldn't get it out of there and into his van fast enough. The engine numbers identified it as a factory racer. It had no lights and it had a megaphone muffler. It was missing the body work and instead had one of those one piece fiberglass tank seat combinations complete with metallic flake paint job. It didn't fit very well. The people at the computer shop said that an old man (possibly the dad of the owner) used to crank the BSA up every now and then and ride it around the parking lot. They said it was really loud but it hadn't run in years.
Jerry contacted Dick Mann about the bike who verified that it could have been a factory bike. He said it "could" have been factory but in those days a race bike had to have matching frame and engine numbers. They had spare cases a used to stamp their own just in case of catastrophic rod failure. Jerry shipped it off for a total restoration and I never saw it after that.
All MY actual barn finds have been total crap.