Here are some of my observations:
(1) I have never seen genuine combined surcharges on any of the following stamps though some are listed in catalogues:
Arms stamps, perforated: 1,2,3,5,7,10,14 and 70 kop
Arms stamps, Imperforate: 4,10,15,20,25,35,50 kop 7 and 10 rouble
Postal Savings, War Charities and Romanovs: all values except the 4 kop Romanov which can be found with combined surcharges (though it's rare)
Since these claims allow falsification I should be pleased to receive scans of stamps which require that a stamp should be taken off my "Does Not Exist" list
(2) Some rouble overprints were applied to sheets with authenticating cancellations applied at the time of the original Z overprinting. So it is possible to find combined surcharges with cancellations dated in the first half of 1920 - these cancellations relate to the Z overprint only
(3) Melik-Pachaev was able to get some stamps overprinted with both the Z and the rouble value at the same time or with the Z applied to stamps which already had rouble surcharges. See the image below where the Z is over the 100 rouble surcharge in the left hand block 4 with part ERIVAN "k" cancel. The Z in such Melik-Pachaev varieties is always (I think) type E18:
(4) Combined surcharges with framed Z are much, much scarcer than with unframed Z - this makes sense if the post office in Erivan was overprinting remainders of earlier stocks. Michel valuations do not recognise this difference - perhaps only 1 in 10 combined surcharges are with framed Z. In addition, fewer values can be found with the framed Z. At its most extensive, my stock included only the following values with framed Z;
Arms perforated: 10/7, 15, 20, 25, 35, 50 kop
Arms Imperforate: 2 kop, 1, 3.50 and 5 rouble
Again this list could be expanded if anyone can scan genuine examples of other values.
(4) Though the 1, 3, 5 overprints can be found in violet occasionally (and rarely for the 10 rouble), no combined overprints show the rouble surcharge in violet, though the Z's are found in both violet and black.
(5) Despite all the philatelic manipulation around this issue:
IF the stamp has a rouble face value, THEN the rouble overprint will be 50 or 100 roubles (never lower)