Development of the R7-series was a corporate secret, occurring during the media blitz that surrounded production of the R6-series astromech droid. The R7-unit featured two fixed, wheeled legs and a third retractable leg, and were heavily shielded against electrical discharges. These droids were primarily used onboard E-wing starfighters, and required significant modification to work effectively with X-wing or Y-wing starfighters. A typical R7 unit could hold up to fifteen hyperspace jump coordinates in its astrogation buffer and survive a near miss from an ion cannon.
The two astromech droid R7-A7 who features in the Clone Wars series has been designated as an R7-series droid. The Clone Wars Character Encyclopedia confirms that R7-A7 is indeed a prototype droid, but it is not explained if the droid is a prototype of the post-Battle of Endor R7 model.
R7-D4 has the name "R7," however, he was created prior to the release of the R7, creating a continuity error. However, the initial two symbols of a droids name do not always connote the model series, as they are part of a longer name, so this is possibly still accurate.