45 heads

[ixtee]

Are ALL aluminum 45 heads marked 5 or 6, or could there be not marked examples....

[RUBONE]

Are ALL aluminum 45 heads marked 5 or 6, or could there be not marked examples....

Early heads are not marked 5 or 6. Look underneath them to see if they have cast in bars/ribs at the outside corner. That designated compression ratio.

[Pascal]

To add; Polish made/ cast repop heads often don't have numbers too

[RooDog]

Polished heads, or Polish heads from Poland?

[RUBONE]

Also keep in mind that the numbers only mean anything if the heads have never been messed with. Many older heads have been milled for higher compression or to repair a poor gasket surface.
This picture is the combustion chambers on a #5 (black) head and a #6. There are also the WLDR and WR heads which differ again. The Aluminum heads stared with the mid '39 WLDD. It was after the military began to spec them that the numbers on the side appeared.

[Pascal]

Polished heads, or Polish heads from Poland?

The latter

[RooDog]

Polished heads, or Polish heads from Poland?

The latter

Much like the difference between nether lands, as in down below, and Netherlands, the nation.