1966 Buick GS 401 Engine

I just purchased a 1966 Buick GS 401 and the engine code on the engine block front reads from right to left MFR (the F may be a t stamped twice) space W340. Can anyome tell me why I have 3 codes and does the 340 stand for the rare 340 horsepower engine? Also my protect-o-plate engine code reads MR340.

 

MR = engine code
340 = production code
:beer

 

I would think the first time the R was being stamped the tool was not held correctly and it was missed stamped with a half a R. Next try was succesful but makes it confusing now.