This writeup and the photo layout are by Shaggy, and he originally posted this on the JJ. I thought it was a really good reference for some earlier pre unit Triumph motorcycle heads, thanks Shaggy!

Here are the 650 heads: Note that they have more of a 'D' shape to them when compared to the 500 heads. They also lack the recess for the spigots found on the top of the 500 cylinders.

There are two basic heads, 4 fin and 5 fin (the little partial horizontal fins by the exhaust spigots don't count). The 4 fin was used on the Thunderbird from 1950-1956 (and possibly early 1957), and the T110 in 1954. The 5 fin was introduced for the T110 in 1955, and then used on Thunderbird 1957 to 1960 (I do have a friend with a very early 1961 6T that still has the iron head)

The main difference between the T110 and 6T heads is that the T110 used the larger, E3310 inlet valve. I have read somewhere that the casting number seen on the bottom of the 5 fin head in these pics is only found on the 6T variant, but I cannot be sure of that:

Here are two of the three basic variants of the iron 500 head. Shown are the pre-war type and the internal oil drain type used 1946-1949. Most important to note is the recess for the cylinder spigot found on all 500 iron heads. That is the easiest way to keep from getting the 500 and 650 heads mixed up.

The pre-war head has the bosses tapped for the external oil drains, and the fins are slightly larger. This is especially noticeable where the fin comes close to the rear oil boss. The internal drain head still has the oil bosses, but they are not tapped. In their place, you can see the two small drain holes on the mating surface of the head.

Not pictured is the 1950-1958 external drain 5T head. It seems to be made from the same casting as the 1946-1949 head, with the smaller fins. (the T100 from 1951-1959, and all pre-unit TR5s use alloy heads)