Hi,
Obviously the signature/marks will reveal all eventually, but one needs to look in the right place.
My first point of call here would be Dartington. Regardless of whether it is acid-etched or sandblasted, a resist or template was used, and quite a nicely thought-about one in my opinion. Individuals have decorated drinking glasses for decades, (small) companies other than those who made the blank more recently. I'd guess that if it was either technique, it would have been a small company, or else a person with access to the facilities neded to execute either technique. Perhaps Dartington know if they sold a batch to a particular person/company for decoration? Particularly if it was an order for a supposed limited edition of 2,500 - more than the average family would buy! If not, maybe they know who did it - the people Eve and I dealt with down there seemed to be very much 'on the ball'. Or else, perhaps they know nothing and the blanks were bought 'blank', so to speak.
I could be horribly wrong, but I think the R.A. is an unlikely target. Firstly, it was the R.C.A. (Royal College of Art), rather than the R.A. (Royal Academy) that Harris, Herman and others in the glass movement were involved with. Yes, the R.A. has probably sold some glass in the past, but I'd veer towards the R.C.A. as it doesn't seem like these came from there to me. Maybe I'll be surprised...
Blimey, I've not been much help! Sorry, but this is just my opinion and is only intended to help narrow down options.
Mark
As a postscript, factory cut and engraved Dartington drinking glasses are very rare. Another person you may wish to try is Peter Robinson, who is the ex-sales Director of Dartington Glass. I don't have his contact details to hand, but he does attend a number of the major glass fairs. It may be out of his period, as he left at some point, but he is certainly worth a go. He's also a particularly marvellous chap!