I believe that that the 5-28-45 negative date is reasonably accurate. Between 1942 and 1945, the facilities at Chama appear to be in a continual state of update and repair. The D&RGW probably made use of the B-7 as temporary office space during this transitional period. During the 3-year timespan, this could explain the need to place B-7 on its own specially laid piece of track.
The recent Chama Yard post from August 4, 2021 by Dave_A outlined (in fantastic detail) the work performed in Chama during it's lifetime, including the 1942 ~ 1945 period.
Try this link for details, I hope it works: Re: Chama Yard