All Auburns with cast iron heads, including 850X, taxi cabs and some early 1935 GF-engines came with Champion C7, part K10341.
All other 8cyl and 6 cyl cars, Non-supercharged came equipped with Champion J6, part K10340
All supercharged use Champion J9, part K10740.
All Champion spark plugs from 1934 up through 1947 had two ribs and are called beer bottle style plugs. These are the most desirable because they came in the cars. After 1947 Champion plugs had five ribs. All had a black or gun blued base. Spark plugs with cadmium bases are 1956 and later and not correct for Auburn.
On top of the spark plug is a brass top K10328, about an inch high with a hex shape on one end which screws onto the plug and wires press into this. All 6 cyl, 8 cyl cars had this and they are natural brass.