The Middlesex Regiment ( 57th & 77th Foot) Other Ranks Bi-metal Issue Cap Badge with Original Felt Regimental Backing Cloth
Most probably a WW2 issue as no evidence of braising holes to reverse. A diamond back badge in the regiment's regimental colours (in equal portions) of gold yellow and dark maroon sewn onto a satin black material backing. The badge backing was in use during WW1 by over forty battalions. Then it dropped out of use, until adopted again during WW2 (source: Badge Backings & Special Embellishments of the British Army p81).
K&K Vol 1 No 672 refers
G48.10
Code: 60027
22.00 GBP