Just checked through Pitmans book on BSA Twins, which was first released in 1960, which quotes the following - "Inspect the oil level and top up to about one inch of the oil-return orifice. The minimum safe oil level is indicated on the outside of the oil tank"
"Never allow the oil level to fall below the oil level mark on the outside of the tank. When replenishing the oil tank, do not fill the tank more than about one inch below the top of the tank, otherwise some oil might seep from the filler cap. It would also render difficult the proper observance of oil flowing back from the engine into the tank"