Exp. Test license plates are used for road-testing of experimental vehicles (or those with experimental modifications). I don’t know when this type started, possibly in the mid 1920s, though the plate pictured below is one of the earlier examples I’ve seen. In the late 1940s and early 1950s, this type had “TEST” stacked vertically on the side of the plate. Later, these plates were blue on white in the standard non-passenger layout with the “Exp. Test” legend. This type is still issued today, on the light blue base with “Exp. Test” screened below the lower bolt holes and all-numeric serial. It is certainly one of the more uncommon types out there.