Salvator Rosa drawing

At Ansorena, Madrid, in a very nice auction, they sell on 20 May 2015 an "Italian School, 17th or 18th century" drawing of soldiers, with a monogram S.R., estimated at 300 Euro.

The high quality of the drawing and the monogram immediately made me think of Salvator Rosa. Sure enough, the same (or a very similar) monogram appears in reverse on some etchings after Rosa. Even better, I've found the genuine Rosa etching with the same image. So, have we found a certain Salvator Rosa drawing, or has someone reproduced the etching in a drawing (which must have taken an awful lot of work and should in itself be worth more than the 300 Euro estimate). My gut feling (which is often wrong) says it is a true Rosa drawing, but please make up your mind or get professional advice if you decide to buy it!

A more certain study for one of the etchings of the same series (the "Figurine" series) sold for $27,400 at Christie's in 2008. That one was more of an early sketch, this one would be a final drawing, ready for the etcher.