Unknown British Tube

This tube is of British origin, and was made in the early to mid 1920's, but it is unmarked, and I have not been able to identify it.  From it's size and internal construction, I believe this to be a low power transmitting tube.  It uses a phosphorus getter, which gives it the red color.  This type of getter usually gives the tube a rainbow coloration (such as certain UV-201A's).  This one, however, seems to have a more even and intense red coloration than most.

The tube is about 2 1/2 inches in diameter at its widest point, and about 5 inches long (including the tip and pins at the bottom).  By comparison, the spherical version of the Type "R" tube is a bit over 2 inches in diameter, and about 4 inches long.

If anyone knows anything about this tube, I would appreciate an email.