MK — a trademark series of the firm, not a year
Marks that can be entered for this maker: Model, type or typology, Model, type or typology, Tang stamp / makers mark.
The introduction year is not documented, so the model narrows nothing.
Sold-price aggregates are not collected yet, so the archive gives no figure — condition, originality of the transformers and a documented recap move this model more than any list price.
Evidence: www.roadshowcollectibles.ca