My knowledge is simply because the river bridge was cast by Walkers of Rotherham, a company for which very little archive material survives. J.G. James was an engineer at the Road Research Laboratory who researched early iron bridges in his spare time, published several papers and wrote in a footnote 1979:
“I am indebted to Miss B. E. Robinson, former Librarian at Boston, who in 1969 located the bridge records and allowed me access to them.”
These went to Lincolnshire Archives in 1974 or later, but the catalogue does not mention Walkers, who are probably somewhere within:
"4. Correspondence with John Rennie and others, 179?-1815 (60 items)"