Excelitas Tech VTB8440BH Photodiode, IR-Filtered
From another member's earlier post, I believe this to be the same sensor used by both the Zone VI Compensating Enlarging Timer and the RH Designs Stop Clock Vario. The part number given is: EG+G Vactec VTB8440BH. Detailed specs are included in the link.
I also own and use the Zone VI unit and the catalog image of the sensor in the above link appears to be a very close—if not perfect—visual match, both in form factor and coloring.
And welcome back to APUG (posting, that is)...