These were sold from the late 1890’s to the 1920's - see below. It looks like the No1 had a focal length of  4.5in for plates of 3.25x3.25in and  6.5x4.75in.

This one has a lever apperture adjustment that works well. Front and rear elements remove easily . There are minor dings to the brass . There is some dust inside and it could use a clean. But likely to take great photographs. Comes with a flange - diameter 2 and 1/8".

Make me an offer if you think the price too high.

From the Vademecum:

Stigmatic Series 11 f6.0 3.5-18in (11 foci) Da019 from 1897It was also listed as 3.25, 4.0, 4.5, 5.3, 6.4, 7.6, 9.0, 10.7, 12.7, 15.1, 18in by Houghton's catalogue. Theysuggested to use 4in or longer for 5x4. They emphasise the good angle covered for cameras with movements.An example was noted as a Stigmatic f6/192mm No74,56x on a 1/2plate Sanderson and others at No69,58xin a Volute shutter on a Sinclair Una and at No77,20x in a dialset Compur. A late version was noted on a SohoPrecision at No168,xxx in a Compur No3.00 million, form the middle 1930's, such as 1935. The choice mayhave been for the convertibility of the Stigmatic since it was an old design by then.These were separable for longer foci, (1.6 and 2x focus), were less costly and sold well so they are easier tofind today. For 5x4, use 6.4in, at f6.0, 4.0in at f16. In use these seem to be good flat field lenses, the imagegaining contrast at f8 and f11, but better than expected, and covering a really wide field. It could still be a uselens. It was made into the 1920's and 1930's. When separated, the front is 2x and the rear 1.5x the focus ofthe whole. It was rated for 70° at f6.0, and 75°-85° at f16. The f6.0 is the commonest of the Stigmatics today,which suggests the sales were often as a professional lens of about 10in, and then it is followed by the Carfac,which seems to have sold in smaller sizes probably to amateurs. The mount of the Stigmatic seems to havebeen revised about 1900 so that it was easier to dismantle to use separate cells. (B.J.A. 1900, p891, for thedesign story see: Photo. Jour. 1897, 21, p167).Series II was still offered in the 1920's in many sizes, with stress on the convertible nature of the lens and thegreat covering power. It was stressed that it was very well corrected for colour and that the front combinationwas fully corrected for astigmatism. Sizes were:Number Focus Format @f6 Format stopped down:!AA 3.25in for 2.5x2in (1/4plate)1A 4in for 3.25x2.25 (5x4in)1 4.5in for 3.25x3.25in 6.5x4.75in2 5.3in for 4.25x3.25in 8x5in3 6.4in for 5x4in 8.5x6.5in4 7.6in for 6.5x4.75in 10x8in5 9in for 8x5in 12x10in6 10.7in for 8.5x6.5in 15x12in7 12.7in for 10x8in 15x15in8 15.1in for 12x10in 18x16in9 18in for 15x12in 22x20in.