This is neat. I just want to share this information I found about R4 compressors at some Australian website (I forgot the name of the website) that sold all kinds of compressors:

V3008 New Compressor: GM/Harrison/AC-Delco R4 with Metric Mount Threads, 12 Volt with Coil at 12 O’clock, with 4.97” Single A Groove Clutch, Gauge Line 1.132”, with Plugged switch port for High Pressure Switch. Supplied with Sealing Washers.

GM / Opel / Vauxhall / Harrison / Delco
Part Number: 1134320

GM/Harrison/AC-Delco R4 Compressors do not have an oil sump and therefore, are very prone to failure, if the system is not one hundred per cent. Please make sure when changing R4 Compressors, that you follow our “Compressor Instructions and Warranty” to the letter. We advise that you never fit a rebuilt R4 Compressor, as they are very difficult to rebuild and from our experience, we have not found anyone who can successfully rebuild a R4 Compressor, to an acceptable standard. Our V3008 is a new Harrison R4, not rebuilt.

Replaces: All GM Standard R4 Compressors with 4.93" Single A Groove Clutch with or without Switch port (for use with Super Heat Switch applications please see our technical bulletin number 1002). Will fit Mercedes, Rover SDI, Land Rover, etc.

Replaces GM part numbers: 1131198, 1131200, 1131340 & 1131382.