I got this photo from one of my 1960s Murano books - different, or should I say more simple form than yours but same idea. It shows they were a 'balancing act' as it were - these are closed off same as your item between base and main body
Blown Glass shape by Giorgio Ferro c1958 - photo copyright Giacomelli 1960
May be wrong track, but I think your item is possibly an older or slightly different version of this?
Something to think about ......