Once upon a time I owned a Pachislo slot which like most units only had a 2 position switch for volume control, low and high.
I purchased a volume control that was recommended within a forum that was popular at the time in order to not damage the amplifier but sold the machine before ever making use of it.
My recall is I paid like $15 or so for this unit which includes a black knob.
If someone has one of these machines and could make use of it, I would be more than happy to ship it to you.
If more information is needed, I will do what I can to answer.
Send PM if you would like the part.