Seems a reasonable question to me, I assume you mean the distance between the inner faces of the shock absorber top mounts?
Here's a thought. The swinging arm for the 51 twin and the 51 Bullet have the same part no., which suggests the sub-frame of the Bullet and twin are the same. Give our hosts a ring, as they have a used '51 Bullet frame in stock, it does look VERY similar to the twin frame, maybe they might measure it for you.
Yes a reasonable question, but you had to assume because of the lack of information, and an answer still wasn't given and Mike I assume is still none the wiser. I'm trying to be helpful as there was no replies.
The rear suspension mounts on the Indian frames are 6mm wider than the English frames on each side.
If you look at the Indian mounts they are offset on the frame tube whereas the mounts on the English frames are central.
Just for comparison, as accurately as I could measure my (2006) Electra-X the distance is a little wider than the early Redditch @ 8&11/16" or 220mm. I also had a quick look at a late (1961) Redditch frame, seems to be nearer 9 1/4".