Roy — What a delightful and interesting acquisition.
1. Colour.
I've not seen Sowerby
malachite like this before. I have long held the belief that Sowerby malachites were made with Opal, a soft translucent white glass, rather than the hard opaque white
Blanc de Lait — a type of glass that you see, for example, used in Davidson's 1960s
Marble. It is unusual to find examples with enough white showing to be able to decide. Your salts are strong evidence in favour of their use of
Opal in
malachite.
2. Cloud or malachite?
... when you first look at these you could easily confuse them with cloud glass as when you hold them up to light they are quite transparent with purple trails ...
There are, to my knowledge, only three ways of mixing two colours available to the maker of pressed glass. Mix the two colours in the pot, which then get further mixed as the gather is taken and dropped into the mould, and further mixed as the glass is squeezed around the mould, and you have malachite / slag glass / Davidson's Marble. Float a small layer of the contrasting colour on top of the base glass in the pot, and take a gather of both, and you have cloud glass, with typical surface trails, just like your salts. Add the contrasting colour to the gather of base glass already in the mould, and the outcome is what we now know as ribbon cloud. Note that your salts cannot be ribbon cloud as there is contrasting colour on the underside of the bases.
So your salts are cloud glass.
However, if you define cloud glass as what Davidson and its imitators produced from 1923, then your salts are not cloud glass.
All this may answer the puzzle of where Davidson got the idea of cloud glass from. Your salts introduce the possibility that Sowerby, the great Victorian innovator in colour and colour combinations, had already experimented with cloud glass, producing a few pieces like your salts, and that it was just such a piece (or its maker) that inspired Davidson in 1923.
3. The pattern — and a mystery.
As James has said, your flared salts are Sowerby pattern no. 1216, and the same pattern with vertical sides is 1215. As Sowerby pattern numbers are sequential, we can date it fairly accurately by reference to patterns 1214 and 1217, which were registered on 22 March 1877 and 31 May 1877 respectively. Pattern no. 1219 was the first of the Crane-inspired nursery rhyme pieces. All the 1215 and 1216 salts I have seen carry the Sowerby trademark.
Now the mystery. Your pattern was registered on 24 November 1887, registered number 87777 — actually it was 1215 that was registered, but the registration would have applied to both shapes. This was more than a decade after its launch. I wonder why? Perhaps a competitor launched a similar design. I've not yet found either shape marked with the registration number, and I usually check.
That's all I can think of, Roy.
These salts are an amazing and important find, Roy, in my not so humble opinion. I am deeply envious.
Bernard C.

Sources:- Cottle,
Sowerby - Gateshead Glass; Stewart & Stewart,
Davidson Glass - a history.