thanks Anne .......... regret I don't know the date of that comment, although I suspect C20, but my opinion would be that those words aren't intended to suggest that in the C19 The Board of Trade openly permitted the Registration of 'other solid substances', in addition to pottery, in CLASS IV. Presumably CLASS Registration appears not to have been policed in the sense that checks were made on legitimacy of substances entered in a given CLASS - in fact it may have been difficult to determine - based on a drawing only - whether a material was glass, metal or earthenware. As we know, it was a legitimate intention that some designs were in fact intended to be made in more than one substance, and on rare occasions comments to that effect can be read in the Archives, but quite how acid etched drinking glasses slip through the net I've no idea.
Being aware of past anomalies - most probably from the C19 - TNA may have considered it a fair warning to researchers that this situation does exist, although it may not have been permitted officially at the time.
Thanks for pointing this out - as you know I've bleated on about mis-classification on many an occasion, and it's not that uncommon for glass items to be in the wrong CLASS. It would appear than earlier workers weren't always aware of this problem, as shown by missing Registration Nos. in most of the reference sources we use.
I've only just fired up again with an internet connection, having been cut-off for a day, so a little behind with my posting.