HIya!
Mine is only cleaner because I've had it longer. It had its major clean up a couple of months ago. About the time I posted about needing to JB weld a bobbin winder, in fact. We have the stuff, we're going to try it this weekend along with twisting up some piano wire to recreate a spring.
I'm not sure what big hole at the back you're referring to. Is it the rectangular one beside the motor controller, in the wood? If so, it matches the one left of the needle on the other side. The 2 are for holding the lid on.
Here are a couple more pics.
The last pic is of the top upside down, showing you the pieces that connect to the base to hold the two pieces together. The yellow string holds my key.
Hope this helps.