Alan,
You're leaping ahead a bit, and blurring the distinction between component manufacturing and component fitting.
You also cannot take a GSR as any sort of proof of proccess. I've been involved in many MoD procurements: GSR's, specifications and official requirements often bear little relation to what actually happens later in procurement and/or production.
As Peter confirms, the forends and handguards were mostly contracted out for production - but that is nothing to do with fitting and assembly.
Parker Hale appears never to have made "L39" spec forend conversions - their own 7.62mm target rifles used a variety of one-off cut No4 forends that usually extend for at least 1" in front of the mid band. Parker Hale also never made its own Monte Carlo butts; these - and most of their sporting rifle woodwork - were made by SILE in Italy, and were bog standard commercial items available from other sources by mail order.
The only newish evidence we have is that Enfieldlock's colleague asserts that the scopes were fitted at Enfield. That is a significant point.