External difference 61x - 6m6? ... 650/701?

[EightySeven550]

Hello all,
I did a quick search but couldn't find the answer.

I am looking at an engine, Is there anyway to tell the difference between a 6m6 and a 61x? There is no intake manifold and I dont see any markings on the cylinders.

Thanks all,
Andre

 

[SUPERJET-113]

On the bottom flange in between the cylinders on the exhaust side are the cast markings 6M6, 61X,62T, etc.