I suppose the back to front issue re the Rd. No., depends on how the mould was made. I always bring to mind the picture in Raymond Slack's book of that ancient looking guy who is shown cutting a mould - but maybe he didn't cut the Rd. No. on that piece. In the same book Slack includes a diagramatic illustration of a mould of a pressed-glass salt cellar - for a swan, c. 1860 - although unfortunately he doesn't give details of whose mould.
I've got three of these small slightly opalescent swans - probably salts - although none carries a Rd. No., so probably more modern C20. Mould lines can be detected/felt down the birds head and neck, but presumably fire-polishing has removed any other seams - fire polishing was a necessity on many of these pressed pieces.
I get the feeling, that of the pressed glass I've seen, the vast majority of Rd. Nos. appear in relief, and the right way round, so back to front I'm sure is in the minority, so presumably there was a specific reason for those few which were back to front.
I seem to recall that the words 'GRACE DARLING' on the stern of the Sowerby boats - whilst on the inside - need to be read from the outside, otherwise they also appear back to front.
The question is were the Nos. created at the time of casting the mould, or might they have been impressed later by the use of hardened steel die stamps.