A milli-ameter in series with a resistor is
a voltmeter. So don't go looking for a voltmeter. This circuit requires a 1 ma meter. The 200k pot makes it a voltmeter with a 200v range (!!). I would suggest that 10K (or 20K) would be more suitable
50ma meter mentioned in the text must be a mistake. With the 200K pot set to it's mid position ( for example) it would require 5000v from the 555 to indicate FSD !
As the 555 can source (or sink) 200ma probably it would work with 50ma meter - if you could find one - in which case the adjuster pot might be 470 ohms. But not recommended !
The 4.7K resistor shown between the output and +v must be a mistake as well. Leave it out.
But what do I know/ I'm just a drummer