A Hay-Mac, eh? First one I have seen. Brockway was also owned by Mack, and used F-Model cabs in their cabovers for at least a while, I'm not sure if Brockway ever had a proprietary cab of their own. For the last 2 years of Brockway's meager existence, they also used R-model cabs in their conventional series trucks.
I drove a couple Brockways, and they were the only trucks I ever despised, until I spent more time in later years around Freight Shakers.