The jack block and its placement on the sdkfz 161/3 "mobelwagen" flak tank... That's a good question! I have this Tamiya kit somewhere in my pile and I do hope to build it some day... and I am sure that this question would come up at that time..., so!
I have to agree with Bish in so far as I have not seen a photo of the mobelwagen which shows a jack block (but I have not seen a lot of photos of this thing, so...) - which would suggest that it didn't come with one...
But my "counter-point" has to be as follows: this vehicle was actually a production item - albeit not a whole lot were ever made. In that it was a panzer IV chassis, and the pics do clearly show these things came with jacks - the standard type as seen on those panzer IV - and typically mounted on the right front fender - it would seem very likely that the Germans would have supplied that jack block to serve that jack. Jack blocks were a standard item on all tanks and apparently on about every derived vehicle made from same, so why there wouldn't be one on this vehicle escapes me! (not, you understand, that I am insisting that those methodical Germans be consistent to a fault in their AFV manufacture...! 
) Saying that this was an experimental "test-bed" prototype does not fit this case.
There are of course some possible explanations: maybe that block was attached on the inside of the hull and accessed by crew thru the crew hatches - no real reason that it could not be placed inside in a space which used to be for 7.5cm ammo or something. This installation would of course not be visible to us modelers - so the parts would be superfluous if this is what happened for real. Maybe they attached it onto the rear hull plate more or less where that muffler for the auxilliary motor which drove the pz IV turret was typically fitted on the gun tank... granted most of the few pics of the rear that I have seen do not show this, but a couple photos do seem to suggest some tray-like fitting may have been there - such as the tray used on the gun-tank to mount this block? A third possibility is that a jack block may have been attached somewhere inside that big deck compartment - front or back low hull wall. It would have been relatively out of the way and yet accessible. (recall that this was considered a vital equipment item, given all the vehicles which did have them firmly attached and available) It may also have been placed into one of those compartments under the deck floor which were accessed by floor hatches? This would again be a place where we modelers won't see it, but...
For what it is worth, I favor the "placed the block in a tray on the rear plate" hypothesis. I feel pretty confident that the Germans would indeed have 1) made sure that a block was carried and, 2) that it was externally accessible to facilitate recovery or repair.
I would post a picture I found to show this, but... have no way to do that, so... see the pic posted on http://www.frontalbum.ru/20818 : it shows two of these mobelwagens from the rear - and what's really cool is that one is an earlier H hull with the cylindrical muffler, and one is a later H or early J with the twin exhaust stacks! Look closely at center or lower left of the rear plate left of those exhaust bits and you will see a tray or bin attached on both vehicles. This bin has been modeled in a build posted on... http://ost-front.ru/2011/10/29/sd-kfz-1613-mobelwagen/
I am pretty sure that the bin in the wartime photo and that modeled on that sharp-looking (IMO, of course! 
) build are for the jack block. It's where I think I'll be placing mine (unless of course someone comes up with more clarification on this little detail)!
Hope this helps!
Bob