gold hall mark

I have a key ring and fob marked Asprey JG&S

The clip which holds the ring is also marked:

JG&S , a picture of a crown, figures 375, an anchor on its side, and the letter N


whilst the fob is marked:

JG&S , a picture of a crown, figures 375, an anchor on its side, and the letter M

Can you tell me when it was made, and why the marks should differ