The Spanish military, like many others, was quite interested in developing a new semiautomatic or selective fire combat rifle after World War II. Franco’s political ties to Germany (combined with Spanish neutrality in the war) gave them unusually good access to German arms designs, and the Spanish experimented with several variations on the StG-44 in the late 1940s and 1950s. Of course, this would ultimately lead to the development and adoption of the roller-delayed CETME, but before that the intermediate cartridge idea was very much a subject of Spanish attention.
This particular rifle, designated Mosqueton CB-51, was one of those intermediate cartridge experiments. It retained the furniture and general appearance of a Mauser bolt action short rifle, but used a long stroke gas piston and rotating bolt in a self-loading action. Only 12 of these were made, although several other designs were also prototyped concurrently (including others also designated CB-51, confusingly).