Well i've searched long and hard tonight for a decent on-line link to put up but can't find anything you'd be able to see to compare on little more than thumbnails.The best for comparison seems to be in Cornwalls book,a sketch called 'he killed his father in a fight'.The k and d in killed are identical to the above.
A couple of points of interest regarding the image of the postcard you put up are that the doodles at the bottom are another point picked up on by Cornwall in relation to doodles in a guest book that sickert apparently drew and secondly WHY Mrs.McCarthy?
One other little snippet the crossed out Sheridan at the top.
Here's a wiki entry on the most famous Sheridan,an Irish dramatist and poet and also a politician,look at the address he was born at!!
Sickert may very well have sent out one, or even several, of the 'Ripper' letters. Nothing I've read about his personality disinclines me from thinking that he was basically anti-social and, bluntly, not a very nice man. He may very well have gotten his jollies by interjecting himself into the investigation. That alone doesn't qualify him as a legitimate suspect.