Images from Moss Europe.
The end of the pump carrying the roller has a rounded side ...
... and a 'machined' side. The flat points towards the rear of the car and slides up past a similar face on the casing. I say 'machined' as this surface looks very rough and irregular indeed.
The flat must be aligned correctly or the roller will be held away from the cam and the pump will develop no pressure. Fit the pump with the output shaft (i.e. prop-shaft if in-situ) rotated to put the lobe of the cam uppermost i.e. away from the pump piston. With the pump fitted if you peer up inside with a good torch you can see the cam and pump plunger, and when the output flange/prop-shaft is rotated you should be able to see the pump plunger moving up and down. Nathan said he had to tighten the pump plug gradually checking the plunger would still move each time otherwise it was catching.
The pump components from the Laycock manual: