Thrum-Eyed Cowslip

Over 60 years ago, in primary school I learned that primroses, cow-slips and ox-slips had two forms called 'Thrum-Eyed' (stamens visible) and 'Pin-Eyed' (stigma visible (looking like a pin-head)). But now I am confused.

It appears that they should now be called 'S-morph' and 'L-morph' but which is which? 'S' for Stamen? or 'S' for Stigma? and what is L' for?

The reason for this confusion is that the website 'Plantlife' is doing a survey (in April and May) of the ratio between the two forms in each location and to participate, I have to get my aged brain to remember which is 'S' and which is 'L'.  What was wrong with the old terms?

Cowslip, Primula veris


Here is a link to the survey if anyone wants to participate:
https://www.plantlife.org.uk/uk/discover-wild-plants-nature/cowslip-survey