This is an early production box of 5,56 x 45 cartridges from the Fábrica Nacional de Palencia, Spain.
Experimentation with this caliber started at the CETME laboratory in the mid 60s. They used Remington military ball cartridges, pulled the bullets and used their own projectiles for the trials. Many of these projectiles were similar to those in 4,56 caliber.
When these experimentals were to be loaded in some quantity for rifle trials, the components were submitted to Palencia for assembly.
Anyway, the cartridges in this box are made from domestic components (Lanza no. 3407, model 1972). They have a cannelured M193-style bullet of gilding metal and lead (3,63 grams), Palencia case and Berdan primer, and a special single-base tubular powder developed at the Granada powder factory for this cartridge. No primer seal and no headstamp.
The bullet has a cut cannelure, non-knurled, in spite of the photo in Lanza’s book.
Some variations of this cartridge are known: Nickel-plated Boxer primer (probably RWS), with or without green primer seal.
The first headstamp in this caliber was applied on May, 15, 1974, according to factory sources, and was (5,56 x 45 SB 74).
When large lots started to be made at Palencia, in 1980, they used 20-round boxes. Anyway this 30-round packaging is no nonsense since most of the 5,56 mm rifle magazines hold 30 rounds. The 30-rounders were the same as the 20-round 7,62 Nato ones, with a corrugated cardboard spacer at the bottom due to the different lenghts.