Here is one of the Penicuik drawings showing British regulars on the march, 1745. Four have snapsacks, and one (2nd from left) seems to have the envelope style:



And here's one of the Morier paintings fromthe 1750s showing grenadiers in field kit. Note the linen haversack worn on the right side, and the dark-colored snapsack (probably leather) worn on the left:



I'd go with a cowhide snapsack...!