I own a GRRW Flintlock Hawken rifle. It was made by Gardell Powell in 1978. It's a .58 caliber.
I know of another five in various collections around the country. At least one of them is a .62 caliber.
How confident are you about the 1976 date, Dan? As you can see in this notice about guns that were stolen in May 1976, they had made less than 10 at that time. According to the information that was given to me by the GRRW smiths making these rifles, none of the first 9 full stock Hawken rifles were .62 caliber. The notice says the two that were stolen were .58 caliber and according to the records the rest were .54 caliber.
If you are correct on the date, could your recollection of the calibers be off, and the rifles you saw hanging on the wall be the two that were stolen?