Charles,
For what it is worth, R8 bodies underwent revisions during the R8 model lifespan, revisions aimed at improving reliability, to the electronics
and the back among other things.
Here is a link that may be worth looking at in its entirety:
http://nemeng.com/leica/005c.shtml
But here’s the Reader’s Digest version:
there have been four generations of the R8 camera body. The R8 in black chrome finish is No. 10081, and in silver chrome is No. 10080. There has been no change in the stock numbers since the R8 was introduced. The different versions generally reflect improvements in the electronic components and their resistance to static electricity. The versions by serial numbers are:
First: SN 2285000 - 2422000
Second: SN 2427001 - 2435800
Third: SN 2464101 - 2477300
Fourth: SN (Higher than 2477300)
For this reason, the stock recommendation is, if you are in the market for an R8 as a keeper, at this point in time, what you should be looking for is a serial number 2477300 or higher.
What you are likely to find is that some R8s are listed for prices which are inexplicably higher than the norm, many of these will be ones with the desirable serial numbers, so, maybe not so inexplicable after all.
Huss mentioned the possibility of the R8 back door scratching film, which is true, but this was one of the defects Leica addressed in later iterations. R8s might be made in Portugal or Germany and this is noted on the door. Early "made in Germany" doors were occasionally replaced by Leica with "made in Portugal" ones as they caused scratches on the film. Or, at least that’s how the story goes.
There are differences R8 to R9, but they were not necessarily worth the money to me, though I’d get the R9 if they cost the same, just due to aging concerns if nothing else.
The R9 is 100 grams lighter than the R8 from using magnesium alloy instead of zinc for the top plate, and aluminium for the base.
There's an additional exposure counter on the top cover of the R9.
On the R9 the back cover display is illuminated and shows film speeds even when set for DX.
The R9 mode selector dial locks in place.
The R9 multi-pattern metering is upgraded to measure exposures in tenths of an f-stop, and the AE lock operates with integrated metering as well spot metering.
The flash-sync settings were also upgraded to take better advantage of ROM lenses. This was probably the most significant change.
There may be some other differences.
Hope some of this might be of use.
After going through almost every Leica SLR model, except the R9, I’ve ended up with two SL2s and two late R8s, (too many? Maybe. Belt and suspenders). I found these to be the two standouts of the entire Leica SLR production range, for me anyway. Leicaflex SL vs. SL2 that’s a tougher choice as there are pros and cons to either.