Spike,
As Carl says this lug was used on the earlier 2 3/4 hp frames and it is so you can take the front barrel off easily. By taking out the top bolt, but leaving the bottom one in place (but slightly loose) and then slacking off the rear engine clamp bolts you can slide the engine back in the frame. This way there is enough clearance for the front barrel to clear the front frame down tube for removal and saves you taking the engine out of the frame just to take the barrel off.
I guess you could do the same thing with the later frames by taking both front bolts out and sliding the engine back and doing the rear clamp up again to stop the engine flopping about without too much effort so is probably why they simplified the design?
-Ian
