My two-bob's worth.
There is a lot of confusion as to what was originally fitted to the "big valve" heads.
I have both, 67-1549 & 67-1571.
The inlets are the same size, 1.455".
I asume that the difference in the heads is the inlet tract as some export machines were fitted with an 1-3/16" carby.
I have attached a page from the factory tuning notes which states that the inlet valves are increased in diameter [1.455"] by fitting valve 67-961 which is listed in some publications as a Super Rocket, '58-'59, 1/16" os, making it 1.5".
I am again assuming that the author means to increase the inlet valve from 1.455" to 1.5".
I think that the Atlantic Green site is wrong in stating that the RGS & Big Valve were fitted with the over size valves as I have never seen a parts book which would back it. Nor the heads in my possession.
Logic tells us that BSA would give a head, with different size valves to standard, a new part number.
My parts book only lists the 67-1551 valve and the book includes all markets machines.
Trev.