[dsraven]

a little more info, like dimensions, may help. have you looked at the possibility that it may simply be a 6" oval stop turn tail light from a grote industries? the ones that mount into a rubber grommet and have a plug in pigtail, they are a sealed unit. then possibly the other holes were used to mount a stainless sheet metal trim part to dress up the area a bit. there are solid backed rubber grommets that would seal out the stuff that flys off the back wheel. anyway, maybe google the product for a quick dimension check of the hole size you have. lots of this style of light were/are being used on home built vehicles and trailers, there are stainless or chrome dress up parts for them as well.