Hi all! This is my mk4 gsr, mace in 1939, along with the filter dated 1938. I've written the history of the mask down in the comments : r/gasmasks
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
New Filters for GSR Gas Mask by British Army
FM12 with Scott GSR filter via adapter : r/gasmasks