The very early Merck frames all have frame numbers under 1000 mostly with no prefix letter with the earliest I have seen been just under 400. The seat stay caps can even with these early ones be either the signature or the company logo. My late 1980/81 is numbered E331 and is only of the earlier ones with the E prefix numbers – yours with a number well over 3000 would be well into 1981 I think. The signature seatstay caps overlapped for a year or two I think with company logo caps. I suspect the E logo numbers start when production was properly established – the earlier frames with no prefix are not at all consistent in their features.