Jack Harrison describes the process in his book, "Traditional Bowyer, More Unnecessary Fun." Basically, Sweetland would start with billets about 36 inches long and about 1 1/4 inches thick on one end tapering to 5/8 inch on the other end. These billets were compressed under heat into 3/8-inch slabs. 3/8 inch square shafts were then cut from these and then doweled, providing a very dense, weight forward shaft. The end of the arrow shaft with the greater density was intended as the point end, providing greater strength immediately behind the point. Of course, it also increased the FOC of the arrow, but no one used that term back then. Of course, the other big advantages of compressed shafts were greater spine and weight in smaller diameter shafts.
Sweetland used other board dimensions as well to yield different levels of compression, spines, arrow diameters, etc. For exmple, I had some 11/32 shafts that tapered to 5/16 a while back that spined at well over 100 pounds and weighed about 800 grains per shaft. I also have some 9/32 shafts that are tapered to 1/4 inch that spine 50-55# and weigh about 475 grains. Still have a few 5/16 parallel shafts as well that spine about 65-70# and weigh about 520 grains, give or take a little. These are raw shaft weights, not arrow weights.
Harrison and a couple of buddies bought the equipment and moved it to Alaska several years ago. I believe Bill Sweetland provided some consultation on getting the equipment up and running. Their company, Alaska Pioneer Supply, I believe it was called, did produce shafts for a while out of Alaskan hemlock. (I think that was the wood species they used.) The business has been for sale for several years.