Under the leadership of M&StL president Lucian Sprague the M&StL's "Mikes" were extensively rebuilt in the late 1930s and, apparently, reclassified M1-46 at that time. Rebuilding for all, or some, included fitting with stokers, disc driver centers, Nicholson thermic siphons, and Worthington feedwater heaters. No. 606 belonged to the group delivered by ALCo in 1915 and was one of two that even received stainless steel boiler jackets and skirting under the running boards. No information is forthcoming about the location, date, or photographer for this image.