Just a "B: model and an MX at that, dont see to many at teh races as they are in the wrong class ( as in pre78 so are out of there leugue in there)
Ask for the serial numbers if theres a 5 series prefix, its most lightly the 75 B, as in there isnt much point in putting a YZ motor or vice versa in there its probally a fair guess its a still complete and correct.