The flat on the shock rod top is more than 6mm and less than 1/4. Nut is not budging on initial attempt and 1/4" wrench is starting to round off the flats. Interesting is snap on offers 3 sizes, seemingly minor differences. Since my wrench is tiny (no leverage) I was thinking of the A137 if necessary (even to assure I tighten new nut enough) - but since my 1/4" wrench is already rounding stuff, this 1/4 socket might also be too big and round the head. https://store.snapon.com/SearchResults.aspx?q=shock+absorber&IsPartsSearch=0&departmentName=Tool+Catalog

Maybe a special volvo tool? Well this is no big deal , i can put the old shock back in , or forget removal and buy a new top mount for new shock, or use heat (explosion?). I was just wondering what's up with these flats and the size. I measured with a micrometer, the flats and the outer diameter in that area as diameter is also included in the snap on tool spec.
measurements:
.240 flat 6.1 cm
maybe .238

.304 in diameter 7.72mm (.3125 = 5/16)

A 6mm wrench won't fit on the flats and i have a old shock tool that must be that 15/64 size as it won't fit. If i just put the original shock back into the car, i'll have a few years to think about all this.