Armenian overprints on Russian imperial stamps probably constitute one of the most difficult fields of Russian philately, especially due to the very large number of forgeries that have flooded the market for decades now. Unfortunately, it would probably be no exaggeration to state that over 80% of all such overprints in the market and in collections are forgeries.
Tchilinghirian & Ashford and
Dr Ceresa have made extremely valuable efforts to describe genuine and forged overprints, as well as provide an accurate classification. Their classification goes well beyond what any catalogue has provided until today, so if one is really interested in expanding their collection in this field, they should get hold of their works.
The drawing that Rod has provided from the Michel catalogue shows that the so-called "unframed monograms" can be roughly divided into small, intermediate and large. Within each of these 3 basic categories there are further subtypes. The situation becomes even more difficult when one has to deal with an incomplete or poorly-printed overprint or with the different stages of wear of the overprint handstamp.

As a rule of thumb, I would never trust the drawings of any of the major catalogues to judge the authenticity of an Armenian overprint. Here are the drawings of the various types of the
large unframed monograms provided by Dr Ceresa:

The overprint on the 35k stamp shown by
jhlovell definitely belongs to the large unframed category, but one would need to compare it with the drawing of the genuine + the drawings/pictures of the 34 or so forgeries given by Ceresa (for the large unframed alone!) to decide on its authenticity.
Not an impossible job, actually quite fascinating, despite the patience and time that it requires.