Most 70's had the 6 speed transmission when it left the factory.
Oliver put a plug ahead of the right shift rail to prevent 5 and 6th. gears from being used on the steel wheeled tractors. All that was needed to be done was remove that plug an you had 6 speeds. Most of the time when rubber tires were added the plug was taken out, so you could have a road gear.
My 70 Hart Parr has had the plug removed and it has never been on rubber.