Who else would know about Tarabar? MAA, maybe Tatiana? I found a very odd letter from Alexei to Tatiana, I think it might be a code. (this is actually how it was written!)
For Tatiana! From Alexei (my addition 27 May 1917 in English)
Old Mother Hubbard
Went to the cupboard
To seek her poor doggie a bone
When she got there
The Cupboard was bare,
And so the poor doggie got none.
As to the speculation that it "might be a code": IMO, I would not allow too much to be read into this, a well-known child's nursery rhyme, dating from the very early 19th century. There are a number of other verses.
IMO, this was undoubtedly written as an example of the Heir's attempt in English, probably copied from a book in English (as opposed to a translation since it "rhymes"). The use of the verb "seek" in the rhyme would to my mind, betray a British English-influenced origin, which given the circumstances of the Heir's origin/s of exposure to the English language, would be appropriate. (Parenthetically, most American children, I think, use "get" in relation to reciting this nursery rhyme.)
But now, even I (as interested in languages as I am) will admit that we are FAR off-topic of the subject heading. AP.