Only a mechanic with the list of part numbers could tell for sure. My first guess might be the wet compass, or the galley coffee pot. The TCAS control head wasn't original in the -200, but the ones installed in those aircraft might still work in a NG; not sure if a Max needs a different one or not. Some other obscure part might fit the bill, but I don't know what. Roller for the toilet paper holder, maybe? Plastic guards on the guarded switches? Individual switches for things like generators and hydraulic pumps, perhaps.
The commonality is less from the -100/-200 to the Max, as it is from the -100/-200 to the "Classic" series (-300 thru -500) to the NG and then to the Max. For each step, it was a common type rating and fairly quick "differences training". The Max has plenty of switches and conventions that are very close to the -200, just not quite identical. For instance, the Max still uses a Master Caution system, rather than an EICAS. Reason? To maintain backward compatibility to the Next Gen aircraft, which didn't have an EICAS in order to maximize compatibility with the Classics. Likewise, much of the overhead panel would be recognizable to a 737-200 pilot, and nearly identical to a Classic overhead. Not "same part number" identical, but very, very close.
A pilot who has flown both pointed out that the 737 NG is a newer design than the 777, even though the latter when it rolled out 20+ years ago was far more automated than the Max is today!
Certainly not simply a marketing device but very real similarities and roots to the past. I doubt many parts from a 50 year old Corvette would fit a brand new one, either, but that doesn't deny the lineage dating back to the original!