I can't escape from the similarity the snail vs knight imagery bears to the knight vs serpent. 'Snake' and 'snail' have the same proto-Indo European root; '*sneg' - meaning 'crawling or creeping thing' . "The word snail essentially is a diminutive form of Old English snaca "snake," (http://www.etymonline.com) The difference with the marginal imagery seems to be the perceivable weaker position of the knights, who in serpent legends are generally victorious. The serpent has been said to represent a number of things - the heathen religion, female sexuality, the yin, the subterranean/unseen, earth energy, etc. Perhaps the symbol was originally a coded heathen statement testifying 'the serpent' had not completely overwhelmed?