Chris King emailed the other day about a year old LF silver cock that grew an extra set of spurs. He came from me as I understand it and was maybe F2 generation after an outcross to buff. Crossbreeding and outcrosses can bring about characteristics neither side of the cross displayed. Often stubs (feathers) on shanks will show up.
Multiple spurs come with age from what I've read, so it makes sense that a breeding cockerel that appears to have normal (single) spurs can start to show or grow more as they become cocks and perhaps even more as they get older. For various reasons some have said they like to use birds that are 2 or more years old to breed from, since some colors and characteristics don't fully express with year old birds and some traits change with age. Multiple spurs may be another reason to add to their list.
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Seller’s genetics site says it is a dominant trait ("Multiple spurs, M, Dominant. Causes more than one spur per shank on males."), so it seems really odd to show up in a breed and variety that doesn’t have it. But these things happen.
I don’t think I’ve ever seen it in my birds, but went out and checked the few LF cocks I have and I saw it in a LF black cock. The 2nd spurs are tight up against the bottom of the main ones and wouldn’t normally be noticeable.