This is an oil seal for the BSA unit singles crankshaft and it is also used on the mainshaft between the final drive sprocket and the mainshaft bearing.
Triumph TR25W 250/441cc (1967-72)
BSA C15/B40 Distributor 250/350cc (1959-64)
BSA C15/B40 Sports Star 250/350cc (1964-)
BSA C25/B25 250cc (1967-)
BSA B25/B44 250/441cc (1967-72)
This used the original part numbers 40-0025 for BSA and 70-8025 for Triumph models.