I was looking at the following Canada 387a St. Lawrence Seaway fake corner number plate block and several things jumped at me regarding it which were missed here.
1) All genuine copies come from post office field stock panes that have the insciptions trimmed off rather than from philatelic centre philatelic stock panes that have intact inscriptions such as found in corner number plate blocks. There are some original corner blocks with trimmed blank margins but no inscribed full corner number plate blocks.
2) The inverted error occurred because this stamp was printed from two different engraved plates where one plate had the blue colour and the other plate had the red colour. The error happened after the sheet was printed in one press in one colour (blue) first and the sheet was then placed upside down (instead of head first, feet first) into the other press which printed the other colour (red).
The fakes are made by cutting out and pasting the centre blue part upside down on another stamp which imitates things wrongly especially for plate blocks. Technically, if the error existed with inscribed plate blocks, the blue coloured inscriptions would face the same exact direction as the blue coloured middle portion of the stamps and would not be upside down like the fakes are. The same applies for the red number 1 in the inscription in that it would also be facing the same direction as the red coloured parts of the stamps. Thus, the red number 1 in the inscription would also be upside down along with the red coloured parts on the stamps and the position of the upside down 1 would be different.
Here is a scan of a fake 387a plate block with blue centres of stamp being upside down compared to blue plate block inscription.
This is how an original 387a plate block would have looked like if it was found from philatelic stock but it was not found this way.
Here is how an original genuine 387a corner block looks like.