Secret marks on Banco de Sonora notes

It was proposed that the notes be marked on either side with miniscule letters B, D and S (for Banco de Sonora).

Banco de Sonora 5 contrasenas face

Banco de Sonora 5 contrasenas reverse
M507 Banco de Sonora $5 marked up

Banco de Sonora 10 contrasenas face

Banco de Sonora 10 contrasenas reverse
M508 Banco de Sonora $10 marked up

Banco de Sonora 20 contrasenas face

Banco de Sonora 20 contrasenas reverse
M509 Banco de Sonora $20 marked up

Banco de Sonora 50 contrasenas face

Banco de Sonora 50 contrasenas reverse
M510 Banco de Sonora $50 marked up

Banco de Sonora 100 contrasenas face

Banco de Sonora 100 contrasenas reverse
M511 Banco de Sonora $100 marked up