[Granitehill]

I can't say for the 300 series (a bit before my time), but I saw - and used - a transparent 700 series (I think it was a 706). It was a sales demonstrator which was shown at public promotional events. Perhaps the transparent 300 series was the same? Under any circumstance, I guess they would be super-rare and very collectable.

As I was working for GPO telephones at the time, I asked if they might be available to the public. The answer was a definite NO. They were specials for demonstration use only...