Hasselblad prism questions

As you say the "E" is for exposure metering. Apart from that the PM3, PME3, PM5, and PME51 are all identical except that the last two "5" prisms have a small cutout for later camera bodies that allows you to see the camera's built in metering display through the viewfinder.

The PME 90 is a 90 degree prism finder with x2 magnification, and the PME 45 is a 45 degree prism finder with x2.5 magnification. Besides higher magnification the PME 45 also allows you to use a polaroid magazine, which you can't do with the PME 90. If you want the camera mounted as high as possible go for the PME 90, otherwise the PME 45 is slightly more versatile, has higher magnification for more accurate focusing and composition, and IMO feels more comfortable with a 503CW and the winder grip.

The PME 90 and PME 45 both have a spotmetering option which can be very useful. Unfortunately this works best with the plain Acu Matte screen, if you use the standard Acu Matte with a central split rangefinder focusing device, it causes problems when spot metering, so you're better off using area exposure metering.

One additional difference between the PME45 and PME90 finders and all the earlier ones is that these two not only offer a spot option (the earlier ones only offer center weighted integral metering), but also have a dome on top, so you can do incident light metering too.

If funds permit, get a PME45. It's the most versatile metered prism of the bunch.
But if you're not looking for a metered one, they will all do equally well.

P.S.
The cut-out mentioned by Gary is not needed for all later cameras, only for the 200-series ones. Your ELX and future 503 do not need it.

I understand the latter series are configured for the current acute-matte screens as no exposure compensation is needed. If you happen to have the earlier screens in your 500 series, you'd need to make some adjustment to obtain a correct read-out for the EV. So make sure the screen amd prism model is matched for this purpose although the accuracy in the read-out may not vary more than 1-2 stops. Alternatively, you may adjust the ASA setting to compensate for the mismatch.

Yes, all the metered prisms from PME3 onwards are meant to be used with Acute Matte screens. Only the original CDS meter prism and the VFC-6/PME are not.
You can of course use all metered prisms with bot non-Acute Matte and Acute Matte screens. All you have to do is set a one stop compensation when an 'unmatched pair' is used.

Thanks everyone for detail information on prisms. I wasn't actually looking for metered prism but it would very helpful for comparison. I guess PM45 would be an idea prism to get. just out of curiosity, what is the magnification facto for PM5? One other question. how can you tell whether the focus screen is Acu Matte or Acu Matte D or non Acu Matte?

Thanks

I believe the PM5 has a 3X magnification whereas the current PM/PME45 is 2.5X. The higher factor naturally makes it a little easier on the eyes for focussing.If cost be a factor you would do fine to get a used metered PME5 over the PM45 series.

Thanks Paul, I am looking at keh.com and they have PME51 but no PME5. Are they same or different?

The PME51 is the newer model and has the same features. It's good value for the money. Ensure the meter reading is not faulty and the battery terminal contacts are not corrosive before you make any purchase. It runs on a single 6-volt battery or 4 LR44 cells which are readily available.

Thanks again Paul, I might just purchased what they got at keh.