The crowned â€œLâ€ is an assayerâ€™s mark used in Lisbon, Portugal, and it guaranteed the minimum fineness of 9 dinheiros, or .750. The specific assayer can be norotiously difficult to identify as their marks were all so similar, but this appears to be one of the marks used by assayer JosÃ© Joaquim da Costa, working 1804-1822. This is in keeping with the makerâ€™s mark â€œAFCâ€, that of Lisbon maker AntÃ³nio Firmo da Costa, registered in 1793. I have no biographical data on him, but his mark is recorded with assayerâ€™s marks used as late as 1822-1843.
Iâ€™m not certain the crab mark is French; it looks a little strange to me, and I can think of no way it could have been legitimately struck on this piece. Lisbon actually used a very similar mark staring in 1938, but it was reserved solely for watch cases. This mark has me stumped.
Hope this helps!
Ref: Manuel Goncalves Vidal & Fernando Moitinho de Almeida, Marcas de Contraste e Ourives Portugueses, v. I: 15th C.-1887 (Lisbon: Impresa Nacional, 4th ed., 1997), pp. 4, 31 & 318; marks 30 & 270.