The quartz sleeve separates the ultraviolet lamp from the water while passing UV-C into it. Length and diameter both identify a sleeve, and this catalogue stocks several at similar lengths — order against the reactor model rather than against a measurement of the old part alone. This is the 505 mm sleeve for the E2, Wedeco part 12754. Note that this catalogue also lists a 505 mm sleeve for the NLR 1536W under item 13179-0513 — same length, different reactor, so order against the model. Wedeco was founded in Herford, Germany in 1976 and is now a Xylem brand.