I did not know Pulmonaria had these pin and thrum configurations. I had always always assumed this was merely a way to force cross pollination, not that pin plants were incompatible with each other, and similarly, thrum plants incompatible within their group.
Since Pulmonaria and Primula are not related, would you know if Primula pin plants are incompatible, and similarly, Primula thrum plants?
That I'm not sure...I have the feeling if you bypass the built-in mechanism to prevent self-pollination, they may indeed set seed. To tell the truth, I never tried to self-pollinate a primula...any crosses I've made have always been thrum (male) to pin (female).
Richards (Primula, 2003) goes on at some length about it in different sections...
E.g.: "...if only one clone of a species is grown (a pin, or a thrum), seedlings raised may be hybrid, and very possibly sterile. However, if legitimate (pin X thrum) pollinations are made with a distantly related species, it is sometimes found that this stimulates the mother to self-fertilize, and seedlings can come true to type. This is known as 'certation'. Such selfed seedlings may often be weak... and it is usually more satisfactory to raise seedings from crosses between pins and thrums. For homostyle species as well...seedlings resulting from crosses, which need to be made by the grower, are more satisfactory than those from selfs, although self-fertilization otherwise takes place automatically. Even when pins and thrums are grown together in the garden, it may be necessary to undertake an intentional crossing programme between the two, if seed is to be obtained... etc." (page 22).
And now, remembering that I too have a primula book (even though I grow very few), I find that many genera are heterostylous. 155 in 24 different families! I guess I have the same Rchards book as you, Alta.
Quite an interesting read (so far), but I think 20 plus pages on the subject might be a little much for me.