Do they sell each other the moulds?
In some cases it's a matter of a company going out of business and another buying up the moulds, in other cases I think they just buy sprues from each other. The former is why Italeri got a range of 1:72 scale AFV kits, for example: ESCI went under 25+ years ago and Italeri acquired their moulds; similarly, Revell appears to have all of the old Matchbox moulds.
Your kit is an example of the latter, but there are plenty more: Tamiya has a ton of kits that also have an Italeri logo on the box, Revell has released not only Italeri but Dragon and ICM kits in Revell boxes, Italeri also release Dragon kits, etc.
That explains why the PE fit so well.
Yep

There's a good number of old kits that were sold by most of those brands, and in more recent times also by Zvezda, that are all the same. Off the top of my head, that goes not just for this QL and the 6-pounder gun, but also many of the Dodge Beep variants and the M3A1 scout car, plus probably a couple of others I don't recall right now.
It pays to look closely at the Scalemates page for accessories for these to see what exactly they will fit.
That got me searching for examples of portees, and of course, there are many; and eventually to this model.
The best-known are this QL for the 6-pounder and other British vehicles for their 2-pounder gun. But I'm sure there are plenty of other examples.
As a way of increasing mobility of artillery in a fast moving mechanized campaign, not as fast as hitching your piece and skedaddling out of harms way
But workable with reasonably light guns. As soon as they got heavier than the 6-pounder or so, it was very quickly impractical.
The alternative, depicted in this configuration, assumes pointing the truck in the exact firing direction with minimal traverse and limiting lower elevation over the cab , and offers a pretty cramped 'working' area for the tending crew.
TBH, firing the gun the way you have it on the vehicle will probably shatter the windshield, and quite possibly the headlights
