3/8" Long Drill Guide

This thing is a drill guide for use with a long 3/8" bit. I am building a farm bridge and need to fasten some 6x6 rails to the bridge deck 3x9" oak planks and I know if I do it by hand it'll never be perfect... and I'd notice it every time I go by. The guide includes alignment marks and something I thought of and have never seen before... windows near the base plate so you can verify your point of penetration and to exhaust filings.