Been following this with interest Dave, and if it didn't come from such a good source it could be the start of an urban myth
The 9308 were around from the '40s and probably thousands were made and trust me the ruby without the larger central bubble are extremely rare. A few of the other colours made including early sea green and sapphire do appear from time to time without the central bubble but the vast majority have the central bubble.
Also another theory that's been mentioned is it was harder to put the central bubble in a ruby one? and because of this and the density of the colour meant you could not see the bubbles very clearly anyway they were omitted?
So to sum up, the vast majority of ruby were made without the central bubble and the vast majority of the other colours were made with the central bubble.