Ben, This particular Kirby Has been In our family since new, My Mother and Father brought it a few month's before I was born, this is the machine that gave me my childhood love of Kirby's when growing up. When I was old enough I got my own H1 I do know that this one didn't have the vent on the nozzle. A few year ago I took the original family Heritage back as my mother struggled with it and I have recently sold my second H1 to a family member in Scotland, I have asked my mother and She is certain that the relief valve was on it when purchased new in 1981, so could this nozzle be left over factory stock modified to fit H1s ?