I believe those are Murphy roofs.
The 3000-series Rio Grande boxcars were extensively rebuilt about this time in the 1920's.
This roughly coincided with the addition of the larger Mikados & other improvements.
Note the rebuild dates in the lower right hand corner of the car side usually matches the new roof date.
I believe that they received steel center sills & body bolsters, etc. at this time.
I've always felt that they look oddly too small when compared to other narrow gauge equipment (especially the later 2-8-2 locomotives)
& wondered why the Rio Grande didn't go for larger cars.
I suppose it was a matter of cost.