Hi,
Rich is of course correct about not knowing for sure when a fowler was fitted with a bayonet. However, we can consider the circumstances under which that conversion was more likely. During the F&I war, Americans fought as British citizens and British troops albeit provincial ones. Certainly some likely volunteered and brought whatever gun they owned. However, most Americans fought as officially sanctioned provincial troops and were issued muskets from British and Colonial stores. They were usually given the old stuff and imported Dutch arms but all would be military guns supplied through ordnance or from commercial contractors such as Richard Wilson and designed to carry a socket bayonet. Moreover, although citizen militia and "trained bands" were active in America since the early 1600s, the British government frowned on them until passage of the 1757 Militia Act. It was that legislation that prompted British ordnance to create the pattern 1757 musket for marines and militia. There probably was no compelling need to convert many civilian guns to take bayonets.
Now consider the American Revolution. Americans fought not as British soldiers equipped by the British government. They had to scrounge up whatever they could and muskets with bayonets were available in limited numbers during the first 2 years of the war. Until we received regular shipments of guns from France and the Dutch, there was a real shortage of muskets. Hence, the "Committee of Safety" muskets and other gun making initiatives. In addition, many civilian guns may have been altered to take bayonets. Consequently, I suggest any conversion of a fowler to take a bayonet was most likely done during the first 2 or 3 years of the American war. It may also have been done to qualify a gun for militia service in the years just prior to the Revolution as relations between Americans and the British government became more hostile and communities armed up, particularly in New England.
Daryl, the pattern 1730 King's pattern musket you show was never meant to carry a plug bayonet. It had a socket bayonet, which is why the stock is cut back. Plug bayonets were long gone when that musket was issued.
dave