The key to the confusion is the number stamped on the top right of the gilded plate. If the number there is preceded by an upper-case letter "B", then you really need to call Yamaha Canada as was already suggested. Or you could e-mail them.
Yamaha uses letters at the beginning of serial numbers only to specify countries of manufacture other than Japan, and the letter B is not accounted for in their list of country codes.
Your customer may have a legitimate concern about the piano's provenance or he may simply be looking for a tool to lower your price. In either case, he's not likely to accept what you learn from anonymous forum participants. Something in the form of writing from Yamaha would be definitive.
B stock refers to seconds, pianos with defects, like scratch and dent furniture. Usually those defects are cosmetic and occur AFTER
a piano is produced and a serial number assigned, most often in packing, in transit, or in unpacking. There is no clear definition of B stock for the piano industry. It's most often something that one dealer accuses another dealer or maker of selling, in other words, a scare tactic.
A manufacturer is not going to celebrate a deficient or defective piano by giving it a special designation in the serial number.
If, on the other hand, the serial number stamped into the top right corner of the gilded plate on your piano consists only of the numbers you provided here with no preceding letter, then you don't need any documentation from Yamaha. You're selling a genuine Japan-made Yamaha from late 1976, The customer can then tell you he wants a discount because your piano is grey market since you're in UK and the piano was initially sold in Canada.
In the US most customers of private sellers expect to knock down the seller's price at least a little, and most savvy sellers set their price accordingly.