The next respirator to appear in the evolution stages following the SR6 is the rather elusive Service Respirator No.8 (S8). It is simply speculated that it's designated as the S8 due to it being featured in a photograph apparently from Porton Down which shows a very rough evolution of respirators used by the British forces from the PH Hood of the First World War to the more recent Service Respirator No.10, more commonly known as the S10. The mask that was never issued in the photo was this mask, so it's likely that the design was finalised and accepted but never actually made it into production, hence why it bears no manufacturer markings.
It is very clear to see how this mask influenced the S10 as it bears several features that subsequently appeared on the S10. Good examples of these features are the inner face seal and the speech module on the side of the mask that allows the attachment of a microphone without any modification, which wasn’t achievable on the S6 because it required a mounting bracket. One characteristic of the S8 that wasn't incorporated in the S10 are the very small eyepieces, although limiting vision more than the ones of their predecessor, the S6, they allowed the mask to easily be used in conjunction with optical equipment such as weapon sights and binoculars. Along with having some common features with the later S10, the S8 also shared features with the S6. The face piece is of a very similar design, the only differences between the two are the eyepieces, speech module and the face seal. Other components, such as the exhale valve assembly, filter mount and the head harness remain identical. Why it didn’t make it to actual production is unknown, however it’s likely that it wasn’t seen as a great improvement over the S6 so all plans for future production were halted and the S6 remained UK's gas mask until the invention of the S10.
These respirators are very seldom seen for obvious reasons, there is one known example in the UK at the Bovington Tank Museum.
Information from www.gasmasksuk.webs.com