About the MK814, the other British steam charging set of WWII
A contemporary of the Alco Firefly is the Mk 814. The Preston Services link below mentions that production of the Mk814 began in 1943 and continued into the 1950s, but I am unable to verify this information.
The design of the Mk814 differs from the Alco Firefly in that it is more robust and purpose-driven. It is much more of a ground-up solution and its construction does not rely heavily on preexisting and repurposed components, which is central to the design of the Alco Firefly.
Stuart produced the engine for the Mk 814. I don’t know if the complete product was a Stuart-only endeavor or if another company was involved. A lot of conflicting and unreliable information exists. Some sources associate it with Richardo. I have no idea what is true. Since it lacks a nameplate, I am confident Arthur Lyon & Co. had nothing to do with this product.
The relationship between the Alco Firefly and Mk814 is worth considering. The two were alternative/competitive products, yet Stuart engines powered both. The design of the Mk814 may have disadvantaged it against the Alco Firefly with respect to manufacturability. Given the pressures and demands of the war, more resources may have been directed to producing the simpler Alco Firefly. If so, one would expect that many more Alco Firefly units were produced than Mk814 units during the war. Once the war ended, priority shifted from quantity to the more polished product. This would explain why production of the Firefly ceased in 1945 and the Mk814 continued for some time longer.
I have read, but cannot confirm, that around 400 Mk814 units were produced, with half of them held in government inventory before being sold as surplus.
Gil Garceau (Connecticut, USA) (Office of Steam, reddrryder on YouTube) owns a pristine example brand new in the crate with all of the accessories, including the oil bottle filled with original oil.
Here is a link to one sold on Preston Services
If you have more information about the Mk814, please write in. My personal knowledge is quite limited about this model.