The safe box were the standards were kept had three locks that were opened using three different keys. These three different keys were kept by three different custodians.

Next to the safe box we can see threee small boxes used to keep the three keys, each one with the corresponding key.

According to Joaquim José da Graça, the custodians were the Ministry of Public Works, the Director-General of Public Works and the Superintendent of Weights and Measures.