This is a 100-round metal box from the spanish firm MMM (Manufacturas Metálicas Madrileñas, Madrid Metal Works). The ammunition division was taken over by Santa Bárbara in 1970.
The MMM ammunition division made ammo in 12,7 x 99 caliber for the Spanish air force only. If I remember well these cartridges were for the machine guns on board of the Sabre jets. The guns were Browning M3 (a shortened version of the M2) and they used M15 belt links. As far as I know, only M8 API and M20 APIT types were loaded.
The cartridges on M15 links should look similar to these ones (these are on M15A2 links for the M85 machine gun).
Here’s a view of the MMM factory from an ad in a 1955 newspaper.