C-18 Smokeboxes

I always wondered why D&RGW 317 and 318 had extended smokeboxes with the little cinder pockets, and my original understanding was that it was a modification the D&RG Made. However, after taking a good look at photographs in my beat up copy of the DHRS 315 book and on the FOC&TS Historic Photo Collection website, it seems that the entire group of F&CC Consolidations had been built with extended smokeboxes, later to have the D&RGW remove them from the ones they received except for Nos. 5/427/317 and 8/428/318. Would anyone know why the D&RGW made this change to most of the class? Why remove them, and why keep them on 317 and 18?