Frank,
You can still open the back door without this piece, the latch isn't that hard to do...
I have the "Key" that you need...it's off a junk/parts chrome F2 that I have...
You do need this part to keep the film chamber light tight...
If the seller can't find the missing key let me know and I'll send this one up to you and any other part you might need off this one...if I still have it or can find it...
The key is four pieces but one unit when assembled and it just screws in from the bottom of the camera...