The 6R-2013 could be 6K-2013
The top right leg of the second character is rounded like an R when zoomed in on the picture. Can't see where that top right leg connects back to the vertical leg, but I'm thinking it's definitely an R.
After looking at the windshield I think it is the remnants of a 1925 Canopy Express.
The windshield is definitely similar in your two pics, but the hood on the OP's truck is longer with very little cowl in front of the windshield. Assuming it's the original hood, it's not the same as the 125 Canopy Express you pictured.
There is a 1918 Vehicle Info Kit, but it doesn't have info on serial numbers. In the last pages there's a discussion on plant production, and the Oakland plant was definitely in operation in '18.
LinkThere's some pics of a Model T, 1 ton on page 100 that looks pretty close to the OP's truck. If it had a worm drive rear axle and solid rear tires, that would be another confirmation.