Have a Ruger 1911 10mm. Want to replace the sights with a night sight. Initial attempt was to buy one from Ruger. It had zero illumination sent it back. Next looked at Trueglo and it's hard to figure out the sizing that would fit my unit. Sizes listed are 260/450, 260/500 and 270/500. Can anyone explain what hose numbers mean so I can determine which one would be the correct fit?