I think the head had failed and someone had switched heads, The 370 had a slide carb the SP400 and GN400 had a vacuum carb, again they can be swapped too.
The other explanation is what Landrover did with the first few hundred Series 2s. They came with the Series 1 half side valve engines
A lot of people say the 370 is the sweeter engine but it's also the weaker. For the 400 Suzuki beefed up the big end and the clutch and gearbox. Least ways that's what I remember. I converted to 12v 25 years ago. It's easy peasy. These days you can buy stuff and stay DC, back then I binned the indicators and battery, and the exhaust. I shoved a 34mm Amal mk 2 on and a protec copy cam, and GN front end. The bike was a revelation after 12 years on a Matchless single. It was reliable and didn't wear out quickly. Least ways the engine didn't. The cycle parts were the opposite. I 'd never changed a wheel or swingarm bearing till I got the SP.