I’m not sure if this will be helpful or not. I did find some letters that look similar to your I-M mark. I also found similar marks to the crown hallmark. However, finding both together wasn’t as successful.
But, perhaps something here might get you a place to start. Also, you’re certain it’s a snuff box? Not a trinket box or another sort of covered piece? Wasn’t able to determine the overall size of the box. Also, have you determined the stone set in the top? That might also give you some hint to location of production.
The various single crowns had portions that resembled the one on the box, however, I’m not sure if the crown’s band is a match or not. The I-M mark looked interesting-again, see if the font is a true match. The 1760’s date would match snuff usages...anyway, let me know if any of this looks promising.
I’ll keep searching and see if I can unearth anything else that you might find helpful.