Very interesting box. At first sight I agreed that that the contents do not belonged to this box, but after examining contemporary examples in other calibers I believe that it could be 100% authentic, including the odd handwriting.
For example, Argentine .303 British contracts of 1948-49 have similar characteristics: 1) light green tip AP; 2) ball ammunition has been found loaded in cases with a tracer headstamp (including red primer sealant) and packed in ball boxes (yellow colour); 3) 50 round box with paper filling.
Also, there is a .303 British 50 rd. export box made in 1948 with handwritting added that reads “PROMOTIONAL STANDARDS” and details the powder type and load used.
Regarding 1954 as the first date when Kynoch made AP loads in this caliber, please note this is the first dated headstamp found in AP, not the first year of manufacture.