If thats is the only place it is and not in the "hole" where the seal fits i would reassemble with no internals fitted and make sure the alloy slider moves freely on the steel leg, If so personally I would reassemble with new seal and try it. One groove is a very small percentage of the "bearing" surface and it ought to be OK. If it really worries you you may get a new old stock slider , or even complete leg from Dave Silver, only if you fit one new complete leg you ought to fit a pair, similarly with used--try and get both from the same bike so all the bits are of a consitent age/wear