Sorry lack of time to read most of the posts in this thread. Your last post Bohdan is an excellent piece of research.
It poses one major question:
Was the drawing done before or after the two etchings?
If the latter then it´s not important.
If the former, then it´s important.
In the curator´s comments of BM 1850,0810.779 they site an anonymous drawing in the National Gallery of Scotland, Edinburgh, as the nearest representation. You will need to acquire an image of that drawing - I can´t find it in their collection database, but then they might not have catalogued everything online.
I tend to think looking at the position of the two left hand torsos in BM 1850,0810.779 that yours would be a copy after the etching, as the third study in your drawing (which is clearly the second torso in the etching) is done in outline only. I don´t know if de Bisschop used a mirror to transfer onto the etching plate (some artist´s did, some didn´t), perhaps we can find out by looking at other etchings of his, because then we would know whether yours would be a copy after the etching due to it being the right or wrong way round.
Director, Pieta Fine Art Ltd
www.pietafineart.co.uk