Having been a vice President of phase 1 I can tell you that those older backs are very nice especially the P 45. You’ll get great image quality and you also find it if you can do very good black and whites with it. There was never a commercially available square chip. At the time that would’ve been prohibitive pretty much as it is today pricewise.
Phase One had made Digital backs for Hasselblad cameras for a very long time. Short history, is that Hasselblad was bought and at that time there was a lawsuit regarding attaching a phase one back for hasselblad cameras, as they wanted a royalty for each back attached. It was a crazy times back in the day. However if you can get yourself a phase one back for your 501 you’ll be very happy with it. A lot of those old backs were tethered only so take a look at what you’re ordering and make sure you can do it untethered.
If you or have any questions feel free to contact me.
CEO & Publisher of PhotoPXL.com and Rockhopperworkshops.com