On the 1.15pm to Liverpool Street, I would have thought this would be a B1 turn by 1957.
Peter Kerslake says "61576 was one of the ten B12/2s built by Beyer Peacock in 1928, numbered 8571-80 and 61571-80 in BR days. The second, 61572 was the last of the class in service, and was withdrawn from Ipswich in 1961.
The load behind 61576 looks formidable, but the B12s were capable engines and had kept things going for over thirty years after the delayed arrival of the "Sandringhams" in late 1928."