The chrome nut doesn't retain the light unit, it simply retains the coloured lens. The light unit is retained by a spring and small horseshoe shaped retaining clip behind the panel. This photo shows how it fits and it's a bugger to fasten!
This, of course, is a repro unit without the resistance wiring. It is also slightly smaller, at 19mm diameter, than the hole for an original unit which is 3/4" - hence the black rubber packing round the hole in the dash.