Tru glo fiber optic - which model?

[POSSUM BEND]

https://www.truglo.com/firearms-turkey/pro-series-magnum-gobble-dot.asp

I'm looking at these sights for my youth model 20 ga 870.  Probably gonna have to order online.   Can anyone verify which model I need?  Website shows 1/4" fits most Remington guns but my rib measures 0.291" with a caliper which is closer to 5/16"? 
Help appreciated before I order. 

[dirt road ninja]

I tried to find the package that mine came in, but couldn't. Off of memory, I believe my 11-87 youth has the 1/4 one. Going to the range this weekend and that will force me to dig thru more stuff, if it pops up I'll revert.