If I remember correctly, when I replaced all the grease nipples on my P I found a couple or so were 1/8" BSP. On the prop shaft?
It is useful to keep a small stock of spares on the shelf as replacements. Also you can use one to inject oil directly into say the rear spring trunnions if you are unsure if the oil gets all the way down the copper lubrication pipes.
YESRERDAY'S COMPONENTS LTD. in chelmsford CM2 8UY tel 01277 840697 have a good stock of straight and angled grease nipples including straight extended for UJ's