I would conclude that the older Martel facility was replaced by a new building in 1941, which then got the name of Universal on the facade. Probably Universal owned a part of the Martel stock and chose to have their own name on the building for promotional reasons. Apparently later Martel became wholly independent again and had its own name placed on the building.
Zenith then bought the entire stock of Martel ca. 1958. The independent stock corporation Martel Watch co S.A. ceased to exist.
Still Zenith never had the Martel name removed from the building in Les-Ponts-de-Martel, although it was their property and it was the place where they produced all the chronographs, including the El Primero. So Zenith did the opposite from what Universal had done : although in 1941 Martel was still a separate entity, Universal put their name on the building; Zenith on the contrary, although they fully owned Martel, let the name Martel be there.