There are clearly regional English differences in the use of "overall/overalls".  In Australia, overall is applicable to the singular, whereas overalls is used generally to talk of the type of clothing, or in the plural. Hence the tip for UK and USA is unhelpful.

The links confirm overall regularly used in the singular:

 

https://www.totallyworkwear.com.au/workwear/overalls.html

https://www.tradiesworkwearshop.com.au/collections/overalls