Nose of Class 31 [TOPS 31-018]
This is the first locomotive of the class and in The National Collection, at Shildon.
Of the 263 produced, some 46 were preserved, but 10 have subsequently been scrapped - one after metal thieves removed the most valuable material.
The discs are the head-code for a "Class D" train ie Express freight, livestock, perishable or ballast train partly fitted with the automatic brake operative on not less than one third of the vehicles.

en.wikipedia.org/wiki/British_Rail_Class_31