carb piston "apertures"

yep. You have a problem. They should both be down flush with the piston base where it curves. I've only seen this a few times.

Well, just my luck! 

Roger, are you saying they both look wrong, or just one?  

What was the solution the other times you saw this? Just replace the piston(s)?

We have never come across this before. It looks like one of the piston slides was machined wrong or is from a different BING Carburetor. Here is a photo of what they normally look like and a rough dimension with a caliper to show you the proper size. 

Please click on this link and submit a CSIR  https://rotaxirmt.com/registration/sir#step-1

I have seen a similar issue on a bing 54.

I can only assume that as these carbs are also used on ground application engines, there are slight differences.....perhaps a slide from a BMW motorbike has found its way into your carb.

I've seen it and just press fitted it back down in place and they have always stayed.