I feel like a putz for offering a dull reply to such an entertaining thread, but I remember when I was a pimply kid outa high school I jagged a job in a camera shop and one day a trailer load of Pentax cameras arrived in the goods-in bay. Obviously the owner jagged them at a 'cheeper by the thousand' price from the importer.
They were MG bodies with this 'pancake' 40mm sold as a standard. I think it performed OK and the rationale was pretty much space/weight saving, so we sold them as... 'ideal for that furtive snap in crowded bizzarre in Marakesh, oh discerning customer sir'... Jeez, I had no shame, even then.
I think (yeah, go ahead- google them and prove me wrong) they had simpler optics, fewer elements and probably didn't 'stack up' (ugh) to the 50mm in high end barrel distortion and CA tests, but hey, they were cheap and popular.
I don't think the name lived on to father generations of great lenses...