Gold coin in pendant (1822 - 1887)

Hi collectors,
I have this piece by inheritance. It is a coin in gold 22k of the reign D. João VI 1822 king of Portugal, Brazil and Algarve (year that Brazil declared independence). But what I like about this piece is the jewelery work of the frame in 19,2k gold (portuguese standard). Two mythological sea creatures to remember the fears of the first portuguese maritime explorers. Overall weight is 22,9 grams (~0,81 oz).
By the hallmarks I know that was made before 1887 but anything more (city, goldsmith). Any old portuguese jewelry expert? xD
Thanks,
joaof