Most of the tube sights I've seen are of the shorter Unertl versions as JDS has described, however Lyman did make one around 16 or so inches long described as a polaroid type. It did contain apertures at both ends. There was also the Kings tube sight (pretty scarce)similar but perhaps a bit longer than the Lyman. I have seen several home made versions with tubes exceeding 24".

Dodging lions and wasting time.....