There are known fakes of the set on the secondary market.
The rams head watermark is just the paper manufacturers branding stamp from the paper making process, like "Hand Made England" etc. its not the publishers blindstamp or estate stamp.
What is the provenance of the etching?
The catalogue raisonne reference of "Song of Songs" set is - Michler and Lopsinger, page 468-79.
It was produced in an editon of 250 on Arches paper, each signed and numbered in arabic numerals, 20 hors commerce lettered A-T, 40 on Japon paper, there were also 6 on parchment in Roman numerals. Which numbering system and what kind of paper do you have with your one? Any editions outside of documented versions above are forgeries. No AP / EA recorded.
The paper size of the authentic version should be 590 x 400 mm. Failing this measurement it will indicate that its a fake.
The originals should have stencil colouring and gold gilding, on top of the drypoints with aquatint technique. Therefore there should be a visible "plate mark" on the paper - an embossed border around the image left from the plate pressure, from the intaglio printmaking process. It should look like this -
http://www.minniesland.com/images/marginalia/platemark210use.JPG
However some fakes do have false platemarks as it is not hard to reproduce.
Does yours have gold gilding and stencil colouring? You can try to have a closer look of the etching using a magnifying glass, intaglio process would not have pixels, and reproductions do. Its hard to tell from photos, I really have to see it in person to judge it.
Hope this helps.