There are some fixed phrases in which adjectives come immediately after nouns. I’m afraid you have to learn them by heart.
Attorney / Secretary General
As to “proper”, when it comes after a noun it refers to the central or main part of something.
Do you live in London proper or on the outskirts?
I visited the city centre proper; it was crowded with a lot of tourists.
When it comes before a noun, it means “real”, “genuine”, “right”, “correct”.
The books should be in their proper place (the bookcase), not on that table.
Eat proper meals instead of junk food.
(The funny thing is that I teach English but I’m Italian).
Hope this helps a bit.
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