J heads diff ? H and M ?

[Dodgy Dick]

I"m looking at a pair of J heads 3671587 out of a truck but one has a "H" following the numbers and the other has an "M", what is difference if any?
Also what chamber volume and year would they be? There dirty and I cant tell if they have steel seats or not?
Over to you once again,
Thanks,
R

 

[heyoldguy]

587 heads are from 1973 and later and will have induction hardened exhaust seats. They will not have hardened inserts unless someone has installed them later.

 

[moper]

The letters are either foundry or core box. I forget which but it doesn't matter. They're all the same. Chambers are open, and usually 3-4ccs la.rger than the factory spec says they are (68-72ccs?).