The first locomotives built there, Arrow and Dart, were used on the opening day of Brunel's Great Western Railway between Bristol and Bath. The owner of the company, who had provided pumping machinery for the building of the famous Box tunnel near Bath, realised that there was a market opportunity providing locomotives for Brunel's new railway. The first locomotives had large seven foot driving wheels and were known as the Firefly class.
Locomotives from the factory were used around the UK and also exported as far afield as Italy, Spain, the Netherlands, India and Australia. At its peak the works employed more than 900 skilled men. Following the closure of the site the company, still trading under the Avonside name, moved out to Fishponds where it continued manufacturing smaller, industrial engines for several more decades.
The derelict buildings survived into the 1990's but were then demolished to make way for the modern offices and flats seen there today.
To find the cache answer the following:
18A1 = year the factory opened
BCD4 = year factory closed
1EF5 = number of locomotives built
Then use the following: 51 2F.BF(A÷E) 2 3E.(C-B)B(D)
The gz is a short walk from the plaque and I have a view of the gz from my office window so hope to see some intrepid cachers! Note added 29/07/19 they have built a building in the way so I no longer have line of sight! Tweezers would help but are not necessary and please make sure the log is sealed to keep it dry.