Nice one Wilxy - would be good to see one of those greetings slips.
Think you are right about the personal nature of the sleeve stamps - like a 'with compliments' slip I suppose for discs sent out on a personal basis. Mine is on an issue copy as you can see from the pic, which also therefore suggests not for a DJ - at least in this case. I agreee it wouldn't make much sense stamping in this way for commercial reasons. Plus, if the use of these stamps had been commercial and widespread there should be loads of stamped sleeves around (given how many near mint, cut-out copies there are of some of the Ric-Tic stuff).
Cheers
Richard