What was the Quillon on Bayonets for? The Hook?

Without an old field manual we are forced to look at other edged weapons. We see that Quillons of this nature were decorative, while top mounted ones were more purpose driven. For reference Levine's Guide to Knives and their Values. We do have a Case Hunter that does break this rule slightly but its non decorative and non intrusive. Some daggers had Quillons, that were forward facing but these were sword catchers, some as long as the blade itself.