While I'm not sure if it is even still available, there is one SD DVR model (2708). If you are ordering an SD DVR, then this is the model you will receive. All other models of DVR are HD.
Perhaps if you match up the rental price quoted it will help you answer the question. I can no longer find the rental price of the SD DVR. The monthly rental price for an HD DVR is $16.99. The monthly rental price for a multi-room HD DVR is $19.99.
Hope this helps.