>>What is closest focusing distance of the various Nikkor 50mm incarnations?<<
For the S-mount lenses, close focus is limited by the camera itself ... 36 inches (0,9 m)and no closer.
Nikon S2, S3, S3-2000, SP, SP-2005 / Kiev 2a
Biogon 21/4.5; CV 21/4; CV 25/4; CV 85/3.5; the following Nikkors: 2.8cm/3.5; 3.5cm/1.8 (1956 and 2005 versions); 5cm/1.4; 8.5cm/2; 10.5cm/2.5; 13.5cm/3.5
Soviet lenses: Orion 28/6; Jupiter-12 35/2.8; Helios-103 50/1.8; Jupiter-8 50/2