T:Turbo
M:Marine
D Diesel
40:series designed in the 60s, so low injection pressure, ergo smoky at startup
A:some differences between A and B
I had a pair of these in my boat, as original equipment from 1980 to 2000. At 4500 hrs I sold them and replaced with 41s. I had some overheating issues, due to the heat exchanger system. By the time I had the 41s waiting, I had solved those issues and was able to sell for roughly what the 41s cost.
My mechanic suggested they were good to at least 10k hours.