Q: I have a 356 cylinder head of unknown origin. It has a stamp of 30/5. What kind of head is this, and how else can I confirm that?
A: Although your Date Stamp of 30/5 infers about July, 1965 (week "30" in year sixty "5") and thus a 912 Head, the Factory did make other heads at the time too.
Here's the quickest way to determine the head type, if the valves are stock and if you have a caliper or dividers and an accurate rule. If the heads have 31mm exhaust valves they are 356A or 356B. If they have 38mm intake valves they are N or S. If they have 40mm intake valves they are S-90. If they have 34mm exhaust
valves they are 356C or 912. If the holes on ea side of the intake ports go through into the rocker chamber they are 356C. If those holes are blind into bosses in the rocker chamber they are 912. Read the numbers off the valves through the spring, or take the valves out. 912 heads should have "ATE 3044" on the intakes, and "ATE 3048" on the exhausts. 356A/B heads have ATE 3030 Intakes or ATE 3043. Sometimes, the first "0" is a "9" if the valve is really original.
912 Heads also have a breather/vent hole about 3/4" diameter, in the top horizontal flat area, a couple of inches away from the intake ports, but this can be added to 356 A/B heads to give the appearance of being a 912 head when in fact it isn't.