Hi Jim,
I don't think it matters for English guns from the 18th century. Certainly, many guns from that period were fitted with pinned keys. For example, my Ferguson rifle (model 1776) is fitted with slotted keys. However, I have seen several English fowlers and pistols without pinned keys. Later English guns usually seem to have them, at least on the better quality ones. Pinning is simple to do even on thin fowler forestocks and the pins can be very small.
dave