I posted a while back that my L5 doesn't have anything to indicate what the oil level in the gearbox should be. I have just seen this on ebay – anyone seen one before "Norton Gearbox Plunger For Harrison L5 Lathe, Part No.L5-3-37
I don't know the lathe but at a guess it's the spring loaded retainer pin that goes in the gear selector arm.
The oil bath should at least reach the biggest gear which will drip it along to the rest. Not so deep as to slop onto your feet when you drop the selector down. Yes been there done that.