Jesse,
My 1955 thru 1967 parts book doesn't show any brake parts starting with number 388 until 1966, so what you have is newer than that as it's not in my book.
The correct number for your 1958 Series 60 with a 16000 pound rear axle is Group 5.017...p/n 3730540...exc, Air Brakes...(Wagner # FD-21956)...fits some 56, some 57 and Series 5, 6, 7, 8 & 10F for 58/59.
Hope this helps,
Mike B
