Promos tended to be---but weren't always---early in the run of pressings. So better quality is a possibility.
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Stamped ones are just probably earlier in the pressing run, and have no extra value. If they have a "Quiex II" sticker then they are rather special, as they are pressed on special, better Vinyl. You can tell if it is really still a Quiex II LP in the sleeve by holding the LP up to the light: Quiex II vinyl is always slightly transparent. I have unfortunately seen some covers marked Quiex II with ordinary vinyl inside.. So you actually have to check the vinyl. Quiex II is worth a premium over the regular LP.
The other great promos are where the LP label has a special label made just for promo copies. These are usually older LPs.. and collectors rate these as also worth more than a regular pressing. The final super special promo is when the LP is a different color, only for that promo. Those are rarer still. And finally, special records never released except as promos.. the record price guides often list those as being worth thousands...