On the pictured Mikado, it is a spare spoke holder., as suggested by SJX426. The Mikado brand consisted primarily of grand touring models and this was a common feature on them. The front braze-on isn't visible in the above photo but here are some photos of another Mikado showing both spoke holder braze-ons.
There have been a number of twin cable rear derailleurs over the years. In the mid to late 1950s both Huret and Simplex offered dual cable versions of their top models. The 2nd cable controlled the cage tension which, at the time, was considered important by professional and avid cyclists, to reduce unneccessary chain friction. Huret used a special duplex cable housing though you could run separate housing.
While Shimano's 1st generation Positron system typically used full length housing, it was possible to run mostly bare cables. This actually improved the accuracy of shifting (as does using modern compressionless cable housing). However, it was technically a single, tandem length cable that was routed from the rear deralleur, then looped around the shift lever spool and back to the rear derailler, to provide a pull-pull system. Cable slippage at the shift lever spool was prevented by a set screw.
On the Mercier, I don't see a 2nd braze-on for spokes, unless it is hidden behind the chainring. While it could be for a 2 cable rear derailleur, the top braze-on is mounted atypically far forward on the top of the chain stay for this function. Normally, the twin cable derailleurs used housings that terminated side by side, both on top of the chainstay, or they ran full length cable housing.