Charles Theodore, Prince-Elector, Count Palatine and Duke of Bavaria (German: Karl Theodor; December 11, 1724 – February 16, 1799) reigned as Prince-Elector and Count palatine from 1742, as Duke of Jülich and Berg from 1742 and also as Prince-Elector and Duke of Bavaria from 1777, until his death. He was a member of the House of Palatinate-Sulzbach, a branch of the House of Wittelsbach.
And as you guessed, the "20 eine feine marck" is a 1/2 thaler:
This is a picture of the digital Krause 1701 - 1800 fifth edition, with only a picture of 1 type of these 4 types. Luckily I also have the Krause 1701 - 1800 fourth edition on paper, and there are pictures of three of those types.
KM# 410 has the date in the legend along the rim.
KM# 411 has the date below in stead of above.
KM# 413 has the three crowned arms in cartouche between branches.
I don't see any cartouche on your coin so I'm pretty sure your coin is the KM# 412 1/2 thaler 1765AS from Pfalz-Electoral Pfalz (previously KM# 123 from Pfalz-Sulzbach)