I'm pretty sure that we're confusing two different shells. Mattis is speaking of the upper one in the attached photo (not my photo; don't recall where I originally saw-and-saved-it). It's 23.5" deep, which is 1.5" short of the rear-column face on a DEC-standard 25" deep rack. Any applicable L-tabs will be short. However I believe that it was designed to attach to the column-sides directly, using a shim, either as identified by KM11 from inside-out (possibly with clearance issues even with a pan-head) or using _short_ 8-32 UNC bolts from the outside-in (rack to the nut-inserts in the shell). IMO the latter is the preferred mounting technique.
Other folks are speaking of the lower one in the attached photo. It's shorter and needs serious-length L-tabs added to reach *anywhere* for attachment. I've seen a variant that is separated into two halves and has no top-sheet. I have at least one set of those, but I also have one like the photo. There may have been a variant that was a full-shell.
Importantly, note the differences in ventilation slots, which corresponds to differences in the design of the chassis that goes inside. The upper-shell has ventilation only on the right-front side, specific to the power supply; the modules are ventilated side-front-back (L-path, unless the applied face includes ventilation openings of some sort -- e.g., under the lip of a protruding control panel on an 11/04), thus "Unibus style" for 6-high modules. The lower shell ventilates entirely side-to-side -- which everyone will recognize is "Qbus style" for 4-high modules :->.