I also have a 77S. Mine was a “Paint-to-Sample” color option that was not offered in 1977. I think you may have the same option. I tried looking up a few sites out of curiosity but as you have, I found nothing. It may take a bit more digging, and btw, did you try searching Rennbow.org?
https://www.rennbow.org/porsche-colors
Any way, if your 77 was a paint to sample it would be a color that obviously wasn’t offered, it may be an old 4-digit code that is not easily found using in your situation, the 3-digit code of 172. This caused some confusion, I’m not sure when Porsche converted older paint codes. It could very well be a color for a vintage 356 or a VW for that matter that the original purchaser was fond of at the time. For instance, my 77 is #018-9-2 (stamped tag #) last available from the factory offerings in 1972 as #6809. If your 77 was not necessarily a Porsche factory color, I would know if they would apply a “172” paint color…say if the original owner wanted the color to match his ‘55 Austin Healy. Porsche would paint new ordered Porsche Any color the buyer wanted with some limits. This non-Porsche color was usually designated as “099” or later as L99 on the paperwork, but the Paint Tag would always and only be stamped with the actual Porsche factory color. Also you can always get a shop to analyze the color to try and match, authorized P shop would have color data.
Lastly, also suspect if your Kunstharzlack tag may even be a replica replacement which are available. . Maybe a typo? Post a picture here and/or search for what an original tag looks like, identifiable by type style of the stamping and rivets and compare with yours. (I’d advise NOT to remove the Kunstharzlack tag!)
A Better location to find original preserved paint in behind the dash gauges and under the carpet, oh and post a pic too.. someone will eventually chime in!
Some of the dual coloring you’re seeing (orangey-brown) could be light undercoats that’s applied before final top coat. Probably your original is in the Brown family.
Here’s another link to get an idea of paint codes :
https://www.porscheknowledge.com/colour-codes-by-year/