Well spotted!
From all I have seen I am unsure whether he acquired the MS in 1911 or 1912. I tend to favour 1911 for the year of acquisition, but the evidence leaves a lot of room for the alternative. If it was 1912, it must have been very early in the year.
Initially, the MS was in Florence before he brought it into the UK, but I do not know if the period in between is weeks or months. In any case we are talking about two different events.
I can see three possible dates for Sowerby's start of work:
- Monday 18 December 1911
- Monday 16 December 1912
- Wednesday 18 December 1912
All are 'incorrect' in one of the four items, and we can be sure about December due to the reference to Christmas. I am not confident that any of the three can be eliminated.
In any case, he difference between the two dates in 1912 is not really relevant, so the question remains on the year.
You will have seen the page where I collect the evidence I am aware of (You are not allowed to view links.
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Login to view.). The summary about the year is section 1.4.1 and the majority of sources point to 1911. ELV's letter is probably the most specific and reliable. This was written in 1931. Sowerby's book was published in 1967, so much later. And they are referring to two different events. Sowerby repeats the year 1912 several times, which may or may not mean something.
To the best of my recollection, sources that cite the year 1912 would be from after Voynich's 1921 publication, so are probably dependent on that.
The book in Cyrillic print may also help, because this was one of Voynich's more notorious finds (at least for himself), and some more information might be available about it.