I am unaware of the meaning of the terms "No. 8 Concert instruments" nor "Solid Black"
Up until the end of 1883 (page 214) the ledger listings only describe the different models, but from the beginning of 1884 (onwards) they are given model numbers.
A "No. 8" was a top-of-the-range instrument, with "Ebony" (or ebonised) ends and 56 keys.
I would suspect that "Ebony" in the ledger may denote an instrument with (possibly laminated?) ebony ends, whilst "Black Solid" may suggest an instrument with solid ends that are ebonised, but couldn't be sure without examining the instruments in question.
I am unable to post a photo here, but will endeavour to PM it to you, Stephen, for your further thoughts ...
So here is (a slightly cropped version of) the photo Malcolm sent me, with the comment "Straight out of the box in natural light, it looks quite black, more so than in the photo, but closer inspection shows what appears to be some very worn "blacking" over what looks to me like rosewood. I've picked the photo where the finish (or lack of it) is most obvious":
Further research has now established that George Roe is unlikely to have occupied the address written inside the concertina until somewhere between the 1881 and 1891 censuses, which implies that it is unlikely that the concertina in question is 18518 (1869) nor 19151 (1875), unless he purchased it second-hand or wrote the address and his initials inside some time after he acquired the concertina, both of which are possibilities of course.
Your photo clearly shows an instrument with laminated rosewood ends that has evidently had its bellows replaced at a later date (seeing that they are evidently 6-fold and these instruments were originally only 4- or 5-fold), and I'd suspect it may have got "blackened up" (and possibly the buttons replaced with "spherical ended" ones) at the same time.
My very strong suspicion is that the instrument in question is indeed number "19151, Jan. to March 1875, 56[-key], Rosewood [ends], Steel [reeds], Unpolished", and that it is the (then newly acquired) 56-key instrument that George Roe wrote the "extended compass" arrangement of his ""Fantasia, Recollections of England" for/on that same year...
So he probably did write the address, and his initials, inside some time after he acquired the concertina.