It seems confirmed that it's a 3.3V LDO, based on connecting up variable power & measuring output. Output was rock steady at 3.3V
With this info, I re-looked at several LDO's datasheet specifically for the 3.3 versions, and found two which matched. Both from Texas Instr - LM3480 & TLV760xx
The package, pin-out, and of course output voltage match !! In addition both are capable of handling inputs upto 30V !! A useful feature for equipment mounted in a car/ truck.
There are of course several other mfgs who make this, but all that I have looked at have a different pinout. (see pic below)
However what did NOT match are the package markings !! So even though I have my answer, I am still curious about the markings. Is it just a case of 'chinese' knock-off ? If so, why put different markings ? Or is there a 3rd IC with these features AND same marking ?
The two Texas IC's seem to have very similar features (quick look), but they ARE different part numbers, and DO have different markings.
Can someone dig up a third IC which has the correct marking ?
I repeat here again -
Marking : MS6R3 (see pic in 1st post)
Package : sot23
function : 3 pin fixed 3.3v LDO
pinout : as per pic below