The plug gages are all in pairs mounted in a common holder
as shown in the upper thumbnail on the page from which you just came.
The largest gage pair is at the left of that image
and at the top of the list below, and the rest follow in order.

Plug1 5/16-18 UNC-3B Go/NotGo in holder; lists at $56.  Price: $7.50

Plug2 #12-24 UNC-2B Go/NotGo in holder; lists at $66.  Price: $8.00

Plug3 #6-40 UNF-2B Go/NotGo in holder; lists at $66.  Price: Sold

Plug4 #4-40 UNC-2B Go/NotGo in holder; lists at $75.  Price: Sold

Plug5 #1-64 UNC-2B Go/NotGo in holder; lists at $83.  Price: Sold

Plug6 #0-80 NF-2B Go/NotGo in holder; lists at $83.  Price: Sold