Here's my gut suspicion . . . the only thing that changed with the 906 heads was the larger 1.74" exhaust valve and the "open chamber". Ports did not receive any significant change. IF there had been something better about the 906 ports (or the '67 440/375 head ports), then all of the car magazines would have mentioned that, but all they mentioned was the larger exhaust valve, back then. Also that the open chamber was supposed to help emissions and "breathe better".
The other thing is that when I compared the heads that came off of my '67 Newport 383 4bbl to a set of 906s, no visible significant difference in the parts, that I could see (looking specifically for that!).
In your heads, the larger valves are already there. I suspect somebody also touched the bowls with a grinder during that operation. Port match at the intake manifold surface (according to NHRA specs) and use them. If you're concerned about port flow/size, then go aftermarket.
The Stage IV heads, of which I have an uninstalled pair, would not have had that casting number, as they came out in the later 1970s. Stage V was more like early '80s, as I recall. Neither were big sellers, from what I remember. What DID sell was when Chrysler had their "warehouse clearance" sale and the '72 heads were priced at $30..00 each, new in the box. THEY went quick! Got a 440-3 short block for $300.00. A box of Mopar fuses, 100 for $10.00? Some other things, too. Those were the days!
CBODY67
