Global Plants is a resource hosted by JSTOR that holds over 2 million images of herbarium specimens (nearly all types) http://plants.jstor.org/stable/10.5555/al.ap.specimen.k000494832. Access is by subscription but it is a great resource to have access. Alterantively New York Botanical Garden has a good online herbarium with images http://plants.jstor.org/stable/10.5555/al.ap.specimen.k000494832 As does Paris Herbarium.
You should ask José Luis Fernández-Alonso, a Lamiaceae specialist at Real Jardín Botánico de Madrid ([email protected]); I am sure that he can provide you properly ID images of this species.
I would be interested in seeing Mentha samples (duplicate sheets, images) to try to identify them. You can send pictures to my email or herbarium specimens the herbarium MA.
I have seen before several American collections of this hybrid spread (fertile materials and other non-flowering). Hopefully I can help.