Hello-
My great aunt passed away and I found this bracelet in her jewellery box. It is very heavy and has the number 750 on the latch. It also has a mark that I truly can't make out. I think it's an "S" or "M" in a box. Thanks for any help!

The number 750 on a gold bracelet would represent that this bracelet is 750/1000 gold, or also known as 18-karat gold.

Here's some more information on gold: All About Gold!

While this is a promising mark, don't hold your breath for too long. Unfortunately there are many creeps out there who love to pass off their work as if it were gold. One way to tell if any metal is pure is to wear it for a day or two. If you notice any odd coloring on your skin, then this means the metal is NOT gold or silver, but is most likely gold (or sometimes even a gold-looking metal, like brass) that is thinly plated over a copper base (untreated copper leaves skin green). Sometimes also when you wear it, no matter how gross it may be, it helps to perspire. ;)

If you are interested enough in this bracelet, invest in having a professional jeweler check it out. Professionals are exceptionally good at identifying real pieces and giving you an estimate on the price. Or, do a little investigation of your own. Was this jewelry box known to have very expensive jewelry, or was this your great aunt's costume jewelry box? Was it especially set aside from other pieces or treated differently than the others (in a special bag, had it's own compartment, etc)?

I can't help you with the S/M mark. It's probably an artist's or company's signature.

Good luck and I hope that it's real gold. It's a very interesting piece!