Likely, P/S stands for power steering. It could stand for power shift, however.
Ford offered an optional, 10-speed "Select-O-Speed" power shift transmission on mid-'60's model 3000 tractors. My father had a 3000 gas with Select-O-Speed for a short time. Select-O-Speed was the first full powershift transmission, to my knowledge. I always thought that they were cool.
Early Select-O-Speed transmissions gave Ford a black eye. They were troublesome. I believe that the problems were sorted out, but the tarnished image remained. Just like International's 560 tractors in the '50's, early problems gave the whole tractor a bad reputation.
If you are interested in the tractor, just ask what transmission it has. If the seller doesn't know, ask where the shift lever is located. If it is on the transmission casing, you know it is gear-drive. If it is up underneath the steering wheel, it is the power shift, Select-O-Speed.