Use of superscript 'x'(?) as an abbreviation for 'yards'

As the OP has already noted in the comments, there are sources suggesting (at least military) use of superscript-x for yards.

Here is a source instructing how to transcribe Navy log books for historical research. It says superscript-x means "yards." (See the "symbols" section near the top of the page; section heading in red text.)

http://www.naval-history.net/OWShips-ForumMat.htm

2600^x - an example of recording distance.

x - Yards. A unit of length equaling 3 feet or 0.9144 meters. Transcribe this as 'x'.

I still remember an ancient notice outside the changing rooms in my high school warning children not to play ball games within 8^x of the goal mouths. It definitely meant yards because I was there long enough to see it replaced with a metric version that said 8 metres instead.

Superscript 'x' for yards was common up to at least the mid 20th Century in British railway signal engineering, which the handwritten text is clearly related to. 'Up home' is a British railway signal designation.

New post carrying H. up home will be provided 70x nearer Stockbridge than exist[ing] which will be [rmvd?]

The drafter of the below diagram evidently could not spell 'therefore':

http://mascil-toolkit.ph-freiburg.de/no/wp-content/uploads/2014/05/Handout-mathematics-in-the-world-of-work.doc