Unless proven to the contrary, then I don't think that's an unreasonable assumption that yours are Blenheim - I forgot now where I'd seen designs that had some discreet extra decoration. On small drinking glasses there's little space for anything other than the basic design, but for bigger stuff I know there have been instances of slight variation, which can be confusing.
The important part of the design is that which occurs on the main body, so think you will be o.k. with Blenheim.