There was one externally obvious design difference within the class with some having a lower running plate which was raised in order to clear the cylinder casing, whereas the remainder had a straight running plate above the driving wheels which made them difficult to distinguish from the later Maunsell S15 when viewed from a distance. The type with the lower running plate had a single straight splasher above the driving wheels in the same form as the N15s.

The earlier built locos had a new style of tender with external bogie frames and large capacity - 5,200 galls of water and 7 tons of coal. N°335 and the 1924 built locos had the more usual "watercart" tenders with 5,000 galls and 5 tons.

A couple ran for a while with saturated boilers for comparison purposes but Urie was a fervent proponent of superheaters and so, eventually, the entire class was fitted with them. With their 21" x 28" cylinders allied with 6' 0" driving wheels and a free-steaming boiler they proved to be excellent workhorses. Indeed, they were so well built that when 30487 was stripped down for general repairs in 1954 it was found that the frames showed little sign of forty years of hard graft.

Twenty six engines were constructed in total in seven batches over a period of twelve years. The first two batches of five in each were constructed in 1914. These were numbers 482 - 491. A further fifteen were constructed in three batches during 1924 with the final one appearing in January 1925. Those later builds received the numbers 521 - 524, 330 - 334 and 473 - 478. The 330 - 334 batch consisted of rebuilds of F13 class locos, this rebuilding was first earmarked by Urie and then subsequently authorised by Maunsell. The 1924 built engines all had the running plate level from the front rise to the rear fall whilst the other members of the class had a rise in the running plate above the cylinders. The mathemeticians amongst you will have noted already that the above notes amount to only twenty five in total. There was an oddity amongst the final quantity, N°335, which brings the final total to 26.

Before Urie's reign started, his predecessor (Drummond) had set aside E14 class 4-6-0 No. 335 for major modifications as a result of poor performance. Urie knew of this state of affairs when he took office and had so little regard for the loco's design that he gained authorisation, under the auspices of a rebuilding order, to use it as the basis of a completely new locomotive. This engine became H15 n335 and was the only one built as a one-off project.

These locomotives were a bit of a test-bed for superheating, something the LSWR was rather late in adopting. Engines numbered 482-5 were fitted with Schmidt apparatus, numbered 486-9 with Robinson's apparatus whilst N°490 and N°491 were built to use saturated steam. The result was Urie's own design, known as the Eastleigh Superheater, that was later fitted to N°490 and N°491. In June 1927 N°491 was fitted with a King Arthur boiler working at 200 lb sq in, and fitted with a Maunsell style superheater. Subsequently the other nine of the above engines were fitted with Maunsell superheaters, but retained their original boilers.
Comparisons of the different types are below: