Improved current mirror
The transistor M1 will generate the bias voltage for the cascoded current source and has no other function.

The following schematic illustrates the point:







simulate this circuit &ndash; Schematic created using CircuitLab

In order to have accurate current mirroring, you would want to have:


The same \$V_{GS}\$, as is the case with all current mirrors
The same \$V_{DS}\$, which is only the case on the right
Matching transistors (layout level)


Adding M3B will improve point 2, by also working on the drain of M5 (M3A in your reference image).