You'll be able identify it from this diagram. On mc18 it's on the left side of the sub frame near the battery
https://www.cmsnl.com/honda-nsr250r7-1990-l-japan_model49620/partslist/0139.html#.W7CHgGBFw0M

Superceded part https://www.cmsnl.com/products/reg-rectassy_31600mv4010/#.W7CIX2BFw0M
_________________
Why do things simply when you can complicate them