0.5x auxiliary Barlow lens

[Slydder]

Hey all,

I just bought a used Euromex scope (60 Euro). The following specs are all I know of atm

WF10X S Oculars
1x and 3x Objectives
33mm diameter threaded auxiliary opening in the turret

I am in need of a 0.5x Barlow that will fit the auxiliary opening and have absolutely NO idea what size I should buy.

Does anyone have time to give a bit of info on this?

Thanks folks,
Chuck