Identification of an interesting 6 cylinder engine

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I've got the opportunity to buy an apparently rebuilt 6 cylinder engine from a deceased estate, the person who unfortunately passed away being the owner of an engine reconditioning business. There are a number of engines, all with new paint, core plugs and clear other signs of work being sold by the family. The price for this engine is a couple of hundred quid so potentially worth a gamble, although it's a 3 hour drive to collect plus a load of hassle at this time of year. It would be annoying to buy it, collect it, and find it was a 2.0 - I need a 2.5.

The seller thinks the engine is a 2.5, but nothing quite matches up in terms of numbering.

I'm in Australia by the way, having left Brighton a decade or so ago - the numbers can be weird here. As are a lot of things.

The engine number is stamped very crookedly (more so than I've seen on most Triumph blocks I've seen) as MK6943HEBW. I can't see this prefix on any of the published lists - although if the block's been decked then it may have been re-stamped incorrectly. Interestingly, I think MK is the suffix for 2500 gearboxes.

The head casting number is 313248 which, according to lists was used across both 2000s and 2500s. The stamped head number is 218226, which shows definitively as an Australian 2500 head - but I can't find any lists which show 313248/218226 as a combo that ever existed.

I'd be interested to know what people think this might be. My guess at this stage is that it's a 2.5 which has been decked and had the engine number re-stamped incorrectly (or the gearbox number was used in error) and that the head is a weird Australian 'whatever parts were available at the time' job.

Some pics are attached - any other observations would be appreciated.

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One more pic: