You should need an adapter.
Keep in mind that we're in different focal-length territory here with the K20D.
On the K20D, the APO-Lanthar will be the equivalent of a 187.5mm lens, which is getting rather longish for portrat work. Probably like shooting the Mamiya with a 300mm on it.
And you're subjects mightn't appreciate the resolving power of some of the sharper glass
, especially on 14.6MP..."Hey, cool, I can see aaaalllll your pores with this thing..."
I don't do much...well, any...beauty work, but I can recommend the K f1.2 50mm...
NOTE: I practically stole that lens.
That's about as beautiful as it gets with me.