Say whut? I have two 503Cx's - same camera with the internal flash sensor .. and there are no shims anywhere. Hasselblad, for years, has made interchangeable screens.... save for the automatic metering cameras (200 Series) that require a cut-out in the screen to allow viewing of the meter display). These new screens work perfectly on the 500 series.
A "Camera Technician" necessary?
1. Remove the magazine and slide the focusing hood/ prism/ whatever is on there - to the rear - and remove it.
2. Depress the two tabs on either side of the viewing screen.
3. Turn the camera upside down and allow the focusing screen to drop into your hand.
4. Install the new/ other screen in the reverse order.