I've looked everywhere, the package is same "length" as SOT-89 (4.5mm) but width is larger ~3-3.1mm, and has only 3-pins similar to SOT-23 instead of SOT-89 3+tab.
2nd pin (middle) is directly connected to ground.
3rd pin (right one) 1V forward voltage drop to ground and 0.5V in reverse. Looks like it's directly connected to +5V from a regulator.
1st pin (left one) no forward voltage drop across but it's connected in circuit, 0.5V in reverse. Reads +5V in operation and goes to MCU reset pin.
The whole board is made in Japan in the 90-00's and uses almost exclusively Japanese branded components. Weirdly this one is branded Microchip. I've even went into checking Microchip's acquisitions thinking it might be a rebranded component.
Came across these but haven't yet checked all variations, the package and brands differ but it could be a copied design.
Any ideas? Thank you
edit: Added pin 1 connection
edit2: MCU Reset pin description: