I fully understand that a certificate must be issued to verify this stamp. Never the less, there is no way I'd spend the money to do so if everyone says "no way". So far I have 2 doubters (term used in a positive sense) and one maybe. I have had some doubts on the ridges myself but I am pretty sure it is a Z grill and if you will bear with me, I'll explain why.
I understand that the ridges are not easily seen on the first post so below are some images scanned at 2400 dpi that might help. The majority of ridges on the stamp are indiscernible even with the higher dpi.
If you look between the right foot of the purple R and left side of the N the pen cancel shows a horizontal black line on two points. The black line is cancel ink absorbed by the ridge. Also below the foot of the T, an uncanceled point and several pen canceled points show horizontal lines. The width between the open end of the "v" in the corners of the point is longer horizontally than it is vertically. That seems to indicate a horizontal ridge.
Leaving the topic of horizontal ridges, I'll describe other features. 85E is the first 12 cent with a grill. That leaves only Z, E or F grills as possibilities. An F grill is easily ruled out due to its 9mm horizontal width. Z and E both have 11mm width.
Z grill 11mm x 14mm 13-14 x 18 points
E grill 11mm x 13mm 14 x 15-17 points
My stamp measures 11mm horizontal and 14mm vertical. That puts it in the Z grill camp. As for points I have 13 plus an incomplete vertical columns. (Incomplete as per stamperix's post.) I inverted the scan of the back image so the incomplete is now on the left. Whether 13 or 14 columns, I'm still in the Z camp. As for rows I have 18 which only the Z grill has on the 12 cent stamp. So, another plus for mine being a Z grill.
The two 12 cent stamps show a more intense black on my stamp but the word is out on my opinion until stamperix comments again.

Next scan is in grayscale; an experiment to see if the grill was sharper.
