That's a clever idea, but wouldn't the length needed be equal to the distance between the speakers? That could be quite a lot of tube!
They had a machine which recorded on wire at that time. Have a look on Dave Rat's YouTube channel. He gives a demonstration of one which he got hold of.
EDIT: Here it is: https://www.youtube.com/watch?v=_qCenGzoTTA
Hence the reason for coiling it up-or it could be run to the location of the speakers (assuming the amp was at the speaker location.
Yes a lot of tube-but money was not an issue.
And consider in the old days how expensive delay units were. That buys a lot of tubing.
Times have change quite a bit. I still remember my first digital delay. You could only select times in 5ms increments (yes 5ms).
It was VERY noisy-so you HAD to drive it real hard (Iright before clipping) and then reduce the gain on the amps to get the noise to an almost acceptable level..
Things are SOOOOOO much easier these days.
But you learn a lot when you don't have a lot of toys to play with