The Pressure Control Coupler was available roughly from 1964 to 1980 and suitable for tractors fitted with Pressure Control. This consisted of a longer inner control quadrant giving a PC section in addition to the Position Control section. There were some differences in the internal hydraulic components as well. With normal hydraulics when you select 'lift' full hydraulic power of 2400 to 3000psi is instantly available depending on the tractor model. With PC the pressure can be varied from approximately 500psi to the full lifting capacity. It is this adjustment of pressure which allows the operator to increase the loading on the rear wheels and therefore gain tractive ability. It is essential that front weights are used to maintain the steering ability of the tractor. PC was a very good concept and quite efficient but was under-used. The explosion of 4WD tractors in the 1970s onwards just about killed PC. I have attached two sheets from my 1975 MF Training School notes to illustrate how the system works. I have a guaranteed unused PC coupler dating from 1965 and have been very pleased to display it on my refurbished 165 until some b****** stole the chain during a lunch-stop on a fund-raising charity road-run. Do you have a suitable tractor to use the coupler on. It was always available as an accessory and no number is given for it in the current parts books. There have been several designs of coupler but are largely all the same for tractors up to 75HP. The plates bolted to the sides can be removed and turned to give the best fit for a particular tractor. There are a number of points to ensure correct fitting such as the angle of the chain relative to the trailed implement drawbar and if a PTO driven implement is used it must not foul the chain when turning.
Hope this helps.
DavidP, South Wales