While its been a while since researching basket hilts, theres nothing like seeing one of these beauties to recharge fascination in them !!!
I have long been fascinated with imbued symbolism in both markings and inscriptions on sword blades, as well as within the construction and elements of the hilts. With the lore of Scotland, there are of course many versions that deal with possible explanations that apply to the weapons.
The unusual shape that in this case presents as a terminal filler joining hilt bars (these sometimes occur as the terminal of the side guard) is termed the 'rams horn' and seems to appear almost in many variations on hilts from the two broad 'schools', Glasgow and Stirling.
Some of these have a degree of imagery of the fluer-de-lis, which has often led to speculation associating France, harboring the exiled Stuart , Bonnie Prince Charlie, which of course, while a romantic notion, is as specious as many of the others often suggested.
The 'S' figures which also occur as fillers between bars and shields have also led to suggestions of Stuart, Stirling, Simpson (for the famed Scottish hilt maker), but as noted these often are installed in reverse, thus seemingly removing the thought of them respresenting that initial. The 'S' also is seen on both Stirling and Glasgow hilts, and on hilts by makers other than Simpson, so that idea seems unlikely.
I'm not sure that any sound explanation is at hand explaining these features, but as always....more research!!
I found references to the 'rams horn' hilt element in "British Basket Hilts" by Dr. Cyril Mazansky, 2005, p.92, 126 and in many other references throughout the illustrations.
It does seem the heart shape often is pierced in these rams horn elements, and in "Scottish Swords and Dirks", (John Wallace, 1970, #26), the author notes that this pierced decoration appears at the end of the 17th century and is used often in hilts throughout the 18th century.
I had thought perhaps the heart might derive from the hilt piercings of this shape found in North European dussack examples of basket type hilts in the late 16th century that were likely used by Scottish troops serving in those regions, but there seems to be a wide time gap between these and the beginning of use.
Another thought that seems interesting, but almost too convenient, is that of the symbols on suits of cards, in those times esoteric but carrying the degree of secret symbolism so popular in that period. The designs found pierced or engraved are often not only hearts, but clubs, diamonds and perhaps spades. The association has been suggested occasionally over time, but no constructive evidence revealed as far as I know.
Just my thoughts and notes from research on these hilts, and I would really like to hear others.