HI ,
I would say your item is a' Mug', possibly even for punch or toddy , yes books can be misleading , B/H states that a Tankard is a footed mug, nonsense, Tankards ,at least those from the 18thc can be with feet and without feet , they can be straight sided , flared or even barrel shaped .all having a handle,tumblers too from the 18thc can be straight sided ,flared or barrel shaped , some even have applied trailing to the bodies , I also have some items similer to yours and much smaller,press moulded with applied handles , the smallest ,around 2 ins tall can have spouts , so a small cream jug I suspect,so i dont think your item can be called a converted anything , add the handle and it becomes a mug or a tankard , I think the size would determine which,interresting things though which is why I have a few ,as Steven said,you can see a transition between blown glass making and press-moulding techniques.
cheers,
Peter.