hi there the weight would be under 10 grams sorry not very accurate but my scales measure a minimum of 10 grams and it gave no reading. or just a fraction heavier than a 8mm stainless steel nut. hope this helps.
Ek-trician, yes M8x1.25 means diameter 8mm, tread pitch 1.25. The 3mm is for the spinner Allem screw.
MissOshkosh, only M8x1-M4. If you need M8x1.25-M4 buy M8x1.25-M3 and ask the service on a mechanic to enlarge the hole and make the M4 tread, there is sufficient material.