Have Rubicon's researchers uncovered any evidence that this version was ever built? My sources indicate that it probably never got further than a prototype, and noone knows whether it had a roof. There seems to be only 1 photo of this vehicle, and that may or may not be genuine.
Meanwhile, there are two similar variants of the SdKfz 251 which definitely did see service, albeit in small numbers - the SdKfz 251/17 (which had a 20mm gun mounted on a pedestal with a crude turret) and the SdKfz 251/21 (which mounted the naval "Drilling" anti-aircraft gun). These are both interesting looking vehicles which would make nice models. Given that you're already producing the SdKfz 250/9, you kind of have this type of vehicle covered, so producing an SdKfz 251 version (especially when it's so obscure) seems a bit redundant.