Hi Ian,
Its what I called the Mk1.5 Merco 61. It had the straight through circular casting (extension of the lower back plate screw housing) as seen on the Mk2 beefed up crankcase casting and the upper backplate screw housing as on the Mk1. I had one and I saw two in marine format, maybe more, but certainly two at the same time!
I never thought much about it at the time, only many years later! I found the carcase of a fuselage in my loft which I thought had said Merco in it. Sadly it was a Mk 1, I did sell some of my 61 s a long time ago and I fear that it was one of those that was the fabled Mk 1.5! While up in the loft, I found much of my old photographic collection. I am sure that I have a photo or two of the Mk 1.5 and I am now wading my way through the last 40 years worth of photos!
If I had realised it was a bit of a special, maybe I would have taken more care of it. I took the trouble to read the Merco history on Dave Day's website. I found that the reason for going from the Mk1 to the Mk2 was to reduce vibration by adding mass (not necessarily the correct solution!). It could be that this beefing up was done in two stages and hence the Mk 1.5? It could be that a few of the 1.5 s escaped captivity. The known 3 were all purchased from the same LHS within a few weeks of each other.
I am confident that I should turn up photographic evidence, I never throw anything away!
Regards,
Andrew.