All welded up
those ports are typically used for syncing the 3 carbs together, when not in use the middle port is just capped off.
I’ve read somewhere that it was the 77’ XS750C (of which only a few models were produced) had a 3 into 1 exhaust, starting the following release XS750D or 2D they implemented the 3 into 2 exhaust
XS750 rotating engine stand
Carburetor : missing gas admission on middle carb
XS750 / XS850 FAQ
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