If am not mistaken, this is one of the style evolution that Paul Bracq was in charge of, conceived between 1959 and 1960. You can see this picture (as well as one showing the front) in the book "Faszination SL" on page 6, published by Mercedes Benz Museum in Stuttgart.
This model was actually built, as opposed to other predecessors to the final production car.
On page 101 of the excellent album "Pagoda Style" prepared and edited by, and published under the savvy supervision of our fellow member Michael Salemi, you will find six drawings representing various concepts of the Pagoda model.
Some additional details as well as a selection of three renderings from 1960 can be found in the book "Les Mercedes Pagoda" (page 31), published in French (sorry Garry), with the foreword of Paul Bracq.
PS. as I posted this message I saw that Michael posted his while I was typing mine