Bracelet with Hallmarks ISBR 750 & Coffin Shape w/Lines across and M on Top

by Lora
(NC, USA)

Sorry, have no way to include photos. Have bracelet, with above hallmarks stamped on clasp. Spiral and approx 1oz.

Trying to find out what coffin shaped hallmark means. No luck so far, would appreciate any info you may have or where to go for it. Thank you.