Newbie question on muffler replacement

[psng]

I have an 87 325i with a build date of 4/87. Looking at replacement muffler in www.realoem.com, the part number is 18101705783 up to 9/87. Search around on the Internet, all aftermarket replacements are either no longer available or out of stock. Only the OEM BMW one is available for special order at www.pelicanparts.com for $ 608. Interestingly though, there are plenty to choose from for 9/87 onward build date (PN# 18301707046). Are the mufflers that different between the build dates? Would the later one fit my older one?

Thanks!

Peter