As originally built the locomotives owed much to Beattie's father, Joseph Beattie's designs and, as such, royalty payments were made under "Beattie's Patent". The driving wheels were of 5' diameter, the cylinders 17" x 22" and the boiler pressure was 120 lb. Most of the class was subsequently rebuilt by Adams at which time the wheel size was increased to 5' 1", the cylinders to 17" x 24" and the boiler pressure to 160 lb. with the replacement of the boiler and chimney by ones of Adams' design. Eventually the chimneys were again replaced with those of the Drummond type.
Four of the locos originally carried names and many were renumbered, mainly due to being added to the duplicate list with the prefix 0 added to their number. Some had new Adams-style cast number plates whilst others had small plates on the driving splashers showing the date of rebuilding whilst N°345 may have had a cancellation line through the pre-existing cab side numbers.