Back again to share some photos and details.
First the specs:
Birkshire
Height: 54.75mm
Base dia.: 22.2mm
Top plate dia.: 22.2mm
cap dia.: 22mm
Tank dia.: 19.95mm
Tank wall thick.: 2.45mm
Tank height: 22.9mm
Total weight:67.10gm
AGA Tiamat
Height: 55.70mm
Base dia.: 22.90mm
Top plate dia.: 22.90mm
cap dia.: 22.80mm
Tank dia.: 19.95mm
Tank wall thick.: 2.45mm
Tank height: 22.9mm
Total weight:82.20 gm
So the Birkshire is slightly smaller, almost insignificant. But is is quite a bit lighter.
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You can see from the photos (Birkshire on left; Tiamat on right) the different taper in the top cap. Also, the center post is markedly different. The top plate is almost identical, except the threading for the cap is finer on the Birkshire. You can also notice the smaller pattern in the knurling of the Birkshire. The base of the Birkshire is shorter than the Tiamat; but the Birkshires's top cap is a bit longer. The positive posts look identical, along with all the screws (not shown) and knurled nuts.
View attachment 151206View attachment 151205
In these photos, it is a little hard to see, but the Birkshire (on the left again) has a bit more stock removed from inside the base. The difference in the center post is more evident.
I will post part 2 which shows close ups of the top cap and base in a little more detail, since I could not upload all photos at once.