If the clutch chaincase cover was fitted with the steel cap/plug,the gear box filler cap/plug was also steel but was plain with no inscription on it,the same also with the aluminium caps/plugs.
below,steel chaincase and gear box caps/plugs.


below,alloy gearbox cap,there is also a slotted version.


below,unknown cap "USE ENGINE OIL" does fit the Triumph chaincase and gearbox , made from steel but knurled edge,which is not typical of Triumph.