Thanks for that, Tom. You've taught me something there. Be that as it may, my French-French legal dictionaries, inc. lexique de termes juridiques, Dalloz, say that - before a French law of the 12 July 1980 - Bon pour was added in handwriting 1. on *unilateral* undertakings like a promesse, gage or a devis
http://www.freelance-info.fr/forum/freelance_10322.html and 2. to stop any alteration of a sum of money paid over. So we are sure that this gage is 1. French vs. Cameroon etc. and 2. postdates 1980?. PS I add - to English-drafted tenancies of UK premises - 'Good as deposit' where an incoming tenant pays nothing but takes over and is unilaterally assigned the deposit of an outgoing tenant who has waived such.