Since I wrote this yesterday, I have tried in vain to find out more about this model. The serial number database gives no clues, at least not for Optimas, regarding the seemingly low number of 13702.
My guess is that this was an earlier, cut-price version of a design which at the top of the range used a high-gloss paint finish, the more familiar, centred metal name badge (rather than decals) and the fancy four-peaked paper support, and that it dates from about 1952 or 1953. Here is a photo taken at Optima's Erfurt plant in late August 1953, showing a couple of models with the flat paintwork similar to mine (perhaps the paper support is simply missing from mine?):
It seems the first Optima portable made - that is, as an Optima - was a version of the early 1930s Olympia Simplex (aka Elite, Progress). This was made by Optima until 1953, long after Olympia had moved on to more stylish designs such as the SM series (see examples of the first Optima portables below).
Optima also continued to make the much slimmer Plana until 1959. Optima remained in Erfurt, which is the capital city of Thuringa and the main city nearest to the geographical centre of Germany.
It would seem the far more common Optima Elite portables, with the high-gloss finish and centred metal name badge, came after this model. They have a more rounded ribbon cover, a much smaller opening at the top of the cover, a silver trim around it, and a new paper support design. The bottom half, however, appears to be identical. Here is one from my collection:
Here is an Optima display, again from about 1953. The Plana and the Elite can be seen in the foreground: