Its Andrews shorthand, its on the top of all bats he makes. My understanding of it is as follows.
The top bit is the customer/retailer name, in this case Pro Direct Cricket by the look of it.
The 3 refers to bat number 3 from this particular batch, with the 4 referring to this batch being 4 bats in total.
The bottom is usually the model, grade and size. P for preastantia i'm guessing, SH is obvious, but the bit in between isn't clear. Is it a low grade? I'm not sure i've seen him refer to anything as grade 5 but thats what it looks like.