MKIV GSR with Type E MkV. The date of the filter, September 1939, is interesting as I didn't think they were making MkV filters this early. Some info in comments. : r/gasmasks
British Army GSR Respirator & Filters Military Gas Mask - Size 3 [JH07 – Pools Surplus Stores
GSR Training Filters For Gas Mask Respirator | #1777922150
British Army GSR Respirator & Filters Military Gas Mask - Size 3 [JH07 – Pools Surplus Stores
GSR | Gas Mask and Respirator Wiki | Fandom
Original British GSR Gas Mask, General Service Respirator+filter+case/bag | eBay
GSR | Gas Mask and Respirator Wiki | Fandom
FM12 with Scott GSR filter via adapter : r/gasmasks