A useful tip from a graphologist mate - although obviously scale isn't taken into account, is to use a transparent, gridded, layer over handwriting samples. In this image, it is clearly seen that Dear Boss and Saucy Jack are by the same hand. It does show angle, spacing, size relative to a comparison sample.
Now from what I understand, reading books etc on the Ripper since the 1970s (Stephen Knight), there's a good chance the letter and the post card were penned by the same person.
What I don't get is coming to the conclusion they are by the same hand by just going on the writing itself.
I know in the 1970s at school I was taught schoolboy Round hand (wikipedia link)
If the class were given a sentence to write out, my version would look very very similar to most, if not all, of the rest of the class. If the teacher collected all the class' Round hand exercise books in, I'd only know mine when returned because it had my name on the front, and maybe a few doodles on the cover!
To me, whoever penned the Dear Boss letter and Saucy Jacky postcard was using Round hand as they'd learnt in school, rather than using their own writing. They didn't want to get caught did they.
These are not clues, Fred.
It is not yarn leading us to the dark heart of this place.
They are half-glimpsed imaginings, tangle of shadows.
And you and I floundering at them in the ever vainer hope that we might corral then into meaning when we will not.
We will not.