The focal length looks to be about 8.8 inches  judging by the Vademecum quoted below and the 1890's advertisment clipped in the photos. It measures 42.73mm  approx across the outside of the threads. The front and back elements unscrew readily for cleaning.  A good lens for taking photos with no obvious significant problems. Examined with magnification and a blindingly bright LED light, there are one or two minute scratches and some cleaning marks on the front surface - and a hint of something about 2mm across between the components of the front lens - perhaps separation. No mold/fungus.

It has a slot for Waterhouse stops, but you will need to provide your own ( a rectangle with hole according to aperture that slides into the slot).

The Vademecum says:

Perken Son and Rayment, 99 Hatton Garden, Holborn Viaduct, London.

In 1889, a specialist agent seems to be G.S.Martin of Birkbeck Institution, nr Chancery Lane.The firm was initially Lejeune and Perken but the lenses seen have all been engraved Perken Son andRayment and this was the trading name in the 1880-1900 period. These are quite common in the UK andoften very attractively finished and engraved but seem to be optically quite conventional. Their adverts. statethey are makers and there are accounts of the factory to confirm it. It is likely that they were also tradesuppliers, and these may not have had their name on them. Thus we have seen an Optimus lens without themaker's name. They were still flourishing in 1900 but were Perken Son and Co by 1914. Incidentally they wereone of the firms which adopted the Voigtlaender trade name Euryscope for their own products. It probably wasused on RR lenses with Jena glass and they may have been the source of some of the quite numerousadverts for Euryscopes in the UK. At least if they were lens makers, it is no longer necessary to assume theywere all imported. At least one may have gone out to France to be fitted to a French 9x12 camera. Thoseseen do not have serial numbers but all have irises and are marked with the intended format.(see also Lejeune and Perken)Optimus Rapid Euryscope f6.0 This was listed by 1889 and was presumeably an RR. Itused "special optical glass", possible ex-Jena? It was made in 5.5in for 5x4, 6x5, 8.75in for 7x5, 8x5, 12in for9x7, 14in for 10x8, 12x10in, ie, 5.5-18in. It was "a most useful lens" according to Traill-Taylor of the Brit. J.Photography. This may also be the "Portrait RR" f6.0 in 12 and 13in focus found later as it was classed as aPortrait lens in the small print in 1889.Fig 008 037 Perken Son and Rayment: (l) Rapid Euryscope for 9x7in and (r) an alloy 'Portable Symmetrical'for 10x8in, and (front) a small WAR .RR Euryscope f7.7 This was made in 9 sizes for 5x4in up to 18x16in, and was seen asa 6in focus.Eurygraphe f8.0 This may be the same as the above( Per002).Optimus Wide Angle Euryscope f9.5 This seems a compromise design, and may be only part way to awide angle. It was made in 3.25in for 5x4, 4.75in for 5x7, 6in for 9x7, 7in for 10x8, 9in for 15x12 and 12in for18x16, with some extra in 1900, eg for 6x5. It may be part way to a "portable" RR as the small print shows butsee also below. It was used by Mr Frith among others.Optimus RR f8.0 5.5 to 25in. The 1906 advert. says it was then 'improved'.It was made in 5.5in for 5x4,6.75in for 6x5, 8.8in for 5x7, 12in for 7x9, 14in for 10x8, 18in for 12x10, 20in for15x12, 25in for 16x18in sizes. (Layout Per002)Fig 031 031 Perken Son and Rayment f8/7in RR in brass with Waterhouse stop set. This may not be theexact product in the list but it is a RR!