As far as I recall, in the reserves there is no hard an fast rule about when you get issued your DEUs and it is more up to the regiment to decide when they want it issued. I know that this year the Ptes with SQ are getting issued highland kit, whereas the ones with only BMQ are not. Previously, it was only the QL3 (BIQ) trained privates that got it, and I've seen other units that issue it the moment you're sworn in, or when you complete BMQ. However, I know other units where trained corporals don't even have DEU, although this is typically more of a supply problem than anything.