blacksleeper

I have an opportunity to purchase some Mercury alum. rect.port heads from a buddy for a price that's hard to pass up. He doesn't know much about them except they were on a 572ci. 2.25" int. 1.88 exh. I've got the casting #'s,and I've checked alot of different sites, but no luck on any info of these heads. Anybody got any info or ideas? Also not even sure what series bbc they came from,whether mkIV, V,orVI? Casting #'s are 900-843425 Thanks.