Hi ,
I too had a good look at this glass and thought Zoude as the most likely candidate , there was also another little wine , i have been to the Museum in Namur many years ago and examples were on display then , even as far back as Barrington Haynes in the 60s these were recognized as Belgian glasses and zoude is known to have started using lead around the middle of the 18th c ,
The hole at the base is to my mind a flaw in production while the stem was being attached to the bowl, this is done by pushing the stem forcefully into the base of the bowl to get a good weld which can result in the nipples and bulges often seen in bowl bases , in this case i think the stem was pushed a little to hard creating a little bubble at the base which has collapsed leaving the knop exposed , the cleaning problem alone would suggest its not deliberate ,date I would think is about right c 1760s possibly a little later,its never possible to be 100% precise.
cheers ,
Peter.