Yes, the Chapin 24-inch Curved Brass Extension - Male (#6-7703) will fit the Chapin Premier 2 Gallon Sprayer (#21210XP).
We apologize but we do not have the specific parts info you are looking for. Chapin and other sprayer manufacturers parts usually are not interchangeable or compatible with other sprayer manufacturer brands and models. You are welcome to order parts to see if they work with your unit, and if they do not, you can return the unused part to us as long as no chemical has been run through it. You are welcome to reach out to any of the sprayer manufacturers directly for assistance with part compatibility.
According to the manufacturer, the Chapin 24-inch Curved Brass Extension - Male (#6-7703) will fit your model #31020 sprayer. You will also need the Chapin Brass Shut-Off Assembly for Poly Industrial Sprayers (6-1898).
Chapin 24-inch Curved Brass Extension - Male (#6-7703) will not fit on the 61900 sprayer per Chapin.
The Chapin 24-inch Curved Brass Extension - Male (#6-7703) would not fit the SureSpray Deluxe 1 Gallon Sprayer (26010). The brass replacement wand for this model is the Chapin Curved Poly Brass Extension Wand with Viton 16 in. (6-7756).
The Chain Slide Pump Sprayer (#6002) will spray20- 30 feet and does come with a 12-inch extension, so you should no need any additional extentions. While the Chapin 24-inch Curved Brass Extension - Male (#6-7703) may fit this sprayer's fittings, you would lose a significant amount of pressure in the sprayer but adding length to the wand. There is a great video of this sprayer in action on the product page linked above.
The Chapin 24-inch Curved Brass Extension - Male (#6-7703) will only work with other Chapin sprayers.