Help on part number location for 1978 R100 OEM Mufflers

I have a like new 78 R100S. The only thing not original are the mufflers. They have nice Keihan ss aftermarket mufflers. I just bought a very nice set of used OEM mufflers to put on and am wanting some help from someone with a 78 model r100 bike with original mufflers. Did they start putting the part numbers on the end of the mufflers by 78? I know the 80's bikes had them on the end.
The ones I have have the part numbers by the mounting brackets and not on the end. The part numbers are 18121233031 and 032. Zeuna, a BMW logo and ZS 120 and 121. Of course, if you go to a BMW parts site such as Max's, you get the new numbers which do not help me. Those numbers are now given for all 74-84 models.
I want the ones on my bike to be like they came originally.
Help if you can.
Thanks

I have a R60/6 ('75), and, yes, Zeuna with part numbers stamped into the end of the mufflers.

The mufflers you can buy new from a dealer with have a PARAGRAPH of emissions information stamped onto them.

The mufflers on my recently sold 78 Motorsport.

Thanks

Thanks for all your help. I think I have it figured out.