SF mobile ovens have almost no mass/insulation outside of the solider bricks, and you can see plain as day in the construction photos and in the way the exterior of the ovens begin their curve right at the door as opposed to the fixed ovens which have a flat area to the sides of the door because of the additional mass. These new mobile ovens have the same flat areas to the sides of the door as the fixed ovens. So what is behind the tiles on the fancy new mobile ovens? Is it a completely different mobile oven design or just a fancy new dress on the same old oven? My guess is that it's a grand facade given the incremental weight required for the mass necessary to fill the shell.
Dear Craig, I am glad you brought up this issue. I have been working with a mobile Ferrara oven at the workplace for five days per week since 3.5 years ago until present. Of course, that does not make me an expert on Neapolitan ovens at all; however, when I converse with those pizzaioli who have been operating fixed Ferrara ovens (of which there is one at a local pizzeria here in San Diego), I perceive differences between mobile and fixed Ferrara ovens in terms of construction, materials used, and performance. Moreover, when I visually scrutinize various construction photos of mobile and fixed Ferrara ovens, I see certain contrasting features that immediately stand out.
Indeed, mobile Ferrara ovens do not appear to be insulated the same way as fixed Ferrara ovens of the same hearth size. I am talking about the insulation around the brick soldier course (and possibly around the dome). Please, see the 1st picture, below, which shows a mobile Ferrara oven under construction. As you pointed out, there is little room left to insulate the brick soldier course that would later encompass the biscotto oven floor. Now, in contrast, take a look at the 2nd picture, below, which shows a fixed Ferrara oven under construction. There is noticeably much more room left to insulate the brick soldier course that would later encircle the biscotto oven floor.
Related to this issue of insulation in mobile and fixed Ferrara ovens is, as you mentioned, the curvature
of the external oven walls to the right and left sides of the oven door. With regard to the mobile Ferrara ovens, the external walls to the left and right sides of the oven door are often "round" (see the 3rd picture below), while the external walls to the left and right sides of the oven door in a fixed Ferrara oven are always "flat" (see the 4th picture below). Perchance, my improvised diagram below (see the 5th picture) explains why the mobile oven has round walls and the fixed oven has flat walls on both sides of the oven door. In re the picture of the mobile Ferrara oven you referred to in your post (the picture is re-posted below as the 6th picture), it seems to be a new mobile model, which does not seem to have the "circular" metal base anymore.
I should point out that, as you are cognizant, this issue of thickness of insulation in mobile versus fixed ovens is not limited only to Ferrara ovens. Many other oven builders have followed suit, but not all.
At last, but not least, I have never seen tuff stones (which seem to have insulating properties) being used in constructing mobile ovens. On the other hand, apparently tuff stones are commonly used, not by all oven builders though, to fence in the domes, internal oven walls, and oven floors of fixed Neapolitan ovens. See the last picture, below, which shows the use of tuff stones in construction of this Agliarulo fixed oven. According to a research paper published by MIT, ". . . Masonry walls made of tuff stone is more durable than concrete and brick walls. . . . Tuff stone is very porous in nature so its density, thermal conductivity value and P-wave velocity is lower than other rocks. Therefore, tuff stone is a good heat insulator due to its porous structure." Source: http://web.mit.edu/afs.new/athena/dept/cron/project/concrete-sustainability-hub/Optimized%20ICF/Background%20Research/2006_Ozkahraman.pdf
Considering all the above, my assumption
is that mobile Ferrara ovens (other than the new model, of which I know nothing about) immoderately dissipate or waste thermal energy, which is perhaps why the external walls of the mobile Ferrara oven at my workplace is usually quite hot to the touch, or is substantially hotter than the external walls of the fixed Ferrara oven at the local Neapolitan pizzeria here in San Diego. Good day!