You are correct, the end covers were made in stamped steel and aluminum diecast, but both SU and Hardi had both. The older pumps had diecast while sometime in the late 70's or 80's, the covers were changed to stamped steel.
The early SU square body did not have the spring on the lower valve (AUB581), this was introduced later, maybe in response to Hardi having that spring ? The SU I have dated Dec 1954 does not have any springs and this is borne out by early
pump cross-section views online. I am currently in the process of adding this spring and also making the parts to modify the valve to add the secondary spring to the outlet valve disc.
This first pic shows the Hardi valve assembly. The domed disc has a small spring underneath pushing down on the normal outlet brass valve disc. The main valve housing is basically identical to the SU but has been machined deeper to account for the additional domed disc.
There is also the normal brass disc and larger spring below the valve creating the inlet valve.
This pic shows the point setup:
Here is a pic from the Healey BT7 parts manual showing no springs exist on the SU pump.