I assumed that it would be the following: http://uk.rs-online.com/web/p/d-sub-connector-dust-caps/6734989/
But, you mentioned that it was labelled as "HD" and it looks like it could be the following: http://uk.rs-online.com/web/p/d-sub-connector-dust-caps/2508134918/
Unfortunately the spec doc...
Apparently the number of pins do not determine the external measurements of the connector, so it could just be a DB25?
It's proprietary I think, it's Epson.
On the back of my Epson GT-1500 scanner I have a port called ADF (Automatic Document Feeder) and it seems to have 26 pins. I read that the most common are 9/15/25/50 D-sub connectors, but 26-pin (DB26?) does exist, just not as common: https://www.thorlabs.com/images/TabImages/DB26_3_F_500p.jpg...