Not the first

LMS Stanier Class 5 4-6-0 number 5000 is preserved in the National Railway Collection, York.

5000 was built at Crewe in 1935 and was initially the first numerically of its class. It however was not the first to be built because the Vulcan Foundry had turned out the first of their simultaneous order, No. 5020 in 1934. It also ceased to be the first numerically when No. 4800 was built in 1944, after the LMS ran out of available numbers after No. 5499 was built.
5000 was built with a low degree superheat domeless boiler.
5000 had 40000 added to its number to become 45000 after nationalisation in 1948 by British Railways.