Hello BGS Mex,
That fuse looks like an original and very weathered from age and exposure. It's been heated up a lot.
I hesitate to call victory yet because a fuse that has been repeatedly overheated. What this old fuse shows means it has had poor pin fit connections in the fuse holder. The fuse holder should be replaced as well.
It is never good to see a voltage difference below 10V when cranking but check against other similar mowers to see what you find.
Simple meter voltage testing and Ohm testing are what tricked you on this one. Yes, it has been a traditional method of testing for years, but there is one better for situations like this.
It is known as voltage drop testing. What is that? How is it performed? How can a meter show a reading on a single wire connection?
It is not a magic trick, but it is often not learned as other methods are.
There are many examples of this online and mostly performed on automotive applications, but the same thing applies to testing any mower starting or charging circuit for a voltage drop issue.
You are NOT measuring from power to ground; you are measuring actual power (voltage potential) loss on a single wire or cable connected from point A to point B. This is an easy way to also test for a poor battery post to cable end connections.
On this Murray mower design connect one meter lead to the battery positive terminal and the other lead to the "B" terminal of the switch, then try cranking (with plug wires off). See what your meter reads when cranking.
Here is an example at a rider battery with a corroded post connection:
www.bing.com
Here is a file from the automotive All Data information that can help also: