Flapping_madly
You ask good questions, no apologies required.
But I do wonder sometimes if Rolls Royce make aero engines these days or just assemble them --they seem to buy in bits from all over the world. Does this strain quality control?
When I was a kid the impression was that they made everything -even all nuts and bolts--from alloys they made themselves in their own foundries. They had, it seemed, total control.
I think the total vertical integration business model dates back to the start of mass production automobile manufacturing. In the US, Henry Ford had and made everything required to produce a car or truck in his own factory.
The same was true during the war years, both in the US and in the UK n the manufacture of aircraft engines. However, with the advent of the jet engine and the globalization or reach of businesses, the vertical model began to fall away. One reason might be specialization where smaller external companies invested both in technology and manufacturing capability to produce certain items of good quality and lower costs than what could be manufactured in-house. Larger companies were tempted to go outside for items as it saved financial resources that would be required to update equipment and manufacturing processes to remain cost competitive. In the end, only the most highly technical items and processes remain in-house. The less technical items can be purchased outside satisfactorily with good quality oversight and planning. Today, this has spread globally as business deals are made to sell product while some of the product content is made in the buyer's country. It does require a good quality organization worldwide and sometimes, on site daily depending on the product being produced. It is also important to have a good technical capability plan and assessment team in place to assure capability before orders are placed. So, yes, Rolls Royce, GE, Pratt & Whitney and others have become more focused on design and assembly of engines and less involved in actual parts manufacturing. The key to success is not skimping on quality oversight no matter where parts are produced.
I will answer the engine question in another post.
I don't know the answer to who produced the stub pipe, but I understand the latest engine revision doesn't have the stub pipe component at all.