Hi Richard,
You are going to be disappointed by XPM. It means cross-pattern material, the sort of stuff you make stowage bins from. ;o) In hangars and stores you might also see doors and partitions made from it.
I HAD vast quantities of pics of Mark 2/3s, particularly from my time with the United Nations Force in Cyprus. They were taken for the benefit of my widowed mother, to let her know that I was having a ball in Cyprus (I'd been slack in writing on a recent Northern Ireland tour, so I made up on this tour). Unfortunately the pics all went to my mother (obviously) and after she died, my brother cleaned the house out and I never saw them again.
Possibly good news is that I have a mountain of negatives filed away that have never been looked at since the day the pics were taken. Having bought a scanner with transparent materials adapter, I recently sat down and looked at a few. Most were of my children, but I did find a reel of Scorpions taken at Lulworth Ranges in 1977. If I ever get my life sorted and find the Ferret pics, I'd be happy to make them available.
As to the question "Is it a rebro Ferret?" I have to say that there is nothing special about them. All they did was carry two radio sets to a suitable location and automagically connect them so that as one received on one net, the other one transmitted the signal on the other net for technical or tactical reasons. All Ferrets were built to carry two radios, so any would do. All rebro Ferrets that I ever encountered were straight Mk1s, but there is no reason why Mk1 variants with extended hull coamings might not be used as rebros. I doubt yours would have been a rebro in the late 70s - early 80s, since in my experience they all had .30" Browning MGs. That doesn't mean that it had not been used for rebro in an earlier time.
As to the tubes on the front plate. Yes, certainly the bottom one was the standard storage position for four-foot Larkspur antenna rods. The military VHF band dictated antenna lengths up to eight feet; military HF worked best with 12 feet of antenna, though eight would usually suffice as the antenna was electrically tuned to the frequency in use.
Clansman brought slightly different frequency ranges, so antenna sections were 1 metre long instead of four feet. Hard for me to guesstimate: is your upper tube just over 1m long? If not, (and assuming the upper tube is military) maybe your vehicle served some other specialised radio purpose. like maybe Forward Air Controller, talking to incoming Fighter Ground Attack aircraft on UHF frequencies and requiring even shorter antennae. Not convinced by this. I was Regimental Signals Storeman for about two years during which time we converted from Larkspur to Clansman. We held UHF sets but IIRC the UHF antenna was incredibly short (inches not feet). Do you know if your vehicle ever served a foreign army? Maybe the upper tube is the same idea, but for an entirely different frequency range. Assuming two sections per antenna, maybe the sets operated about 100-150MHz. But there are a lot of assumptions in that.
Looking at your pic, the front antenna base looks like it's probably Clansman. It might be Larkspur HF (C13) but I'd have expected the HF antenna to be at the rear so that the antenna could be sloped without risking the commander's head. I'll go for Clansman.
We didn't change our antenna storage tubes after conversion, so I am truly confused by your upper tube.
Hope some of this helps.