Be careful with this - I don't know which model you need the part for, but if you look at the parts list (e.g. this one):
https://www.fowlersparts.co.uk/parts...gnal-lamp-note
Part 8 is the lamp assembly which I don't think includes the bit you want. I can't tell which bit you need, but some of them are in stock - e.g. part 13.