The brass jet is most likely a tolerance thing. You can't guarantee a 0.05mm tolerance (possibly even tighter) in plastic, and maintain it over a wide range of temperatures and conditions. Cost would preclude mass producing the parts in solid brass.

Also, off the top of my head, there are three different (STD.) Y pieces... and don't forget there's also HRC variants, that are different again! It's much simpler to have a generic plastic housing with consistently accurate jets pressed in. The numbers in the diagram above are the most common, and will work across the board for standard MC21s and MC28s.
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Andy.
NSR-WORLD.COM

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