I have a 1275 that a previous owner put in my MkII that has the prefix DBL120E. The front plate has the extended ear that like the ADO16. I'm not much help though since I've never been able to find a reference for it.
I think the particular engine I'm asking about is presently in the US, however, Ian, I think you've nailed it - Delta Reconditioned Engines. I did note that the engine tag I posted up here was off-set stamped towards the top where as the factory ones I've seen seem to be a bit narrower and the font is down the centre.