Who else could have written it?

[Ben]

Probably an east-ender trying to sound a bit more "proppa" and official-like, Robert. In much the same way that today's uneducated Londoner might use "myself" where "me" would do very nicely.

[Lechmere]

And dare I suggest that a lot of less well educated people don't know how to spell contractions and don't know where to put the apostrophe.

[Ben]

Absolutely, Lech - that too.

[Hunter]

Victorians were not in the habit of writing contractions, even if some spoke that way.

[Wickerman]

Victorians were not in the habit of writing contractions, even if some spoke that way.

Indeed, writing contractions was 'bad form', unless when quoting someone's speech verbatim. This was back in the day when people & media really cared about how they structured sentences.
:-)

[Adam Went]

Maria:

Don't think there's much doubt that the writing on the wall was reasonably fresh, given the conditions of the East End and how quickly it would have become either partially or completely unintelligible, if not wiped out altogether, had it been there for any lengthy period of time.

However, being a weekend could also be a hint, especially if it was a businessman (or former businessman) who wrote it - if he worked all week and then went out and got drunk on the weekend (as you do), that could have been the time and influence he needed to write the message. Just a suggestion.

Agree with everybody else's interpretation of the manner of speech in London at the time, especially amongst the poorer classes.

Cheers,
Adam.