The crowned “L” is an assayer’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 José Joaquim da Costa, working 1804-1822. This is in keeping with the maker’s mark “AFC”, that of Lisbon maker António Firmo da Costa, registered in 1793. I have no biographical data on him, but his mark is recorded with assayer’s marks used as late as 1822-1843.
I’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.