Latest from Tek on this issue (still ages ago, of course) is that U800 is not supposed to have two nuts on it; there was a TSB on it I remember from when I first started messing with them decades ago. After a certain Ser# they shipped with only one nut on the metal tab, and earlier models sent in for service would have the one not on the metal tab removed. They were also supposed to have the one nut on the tab retorqued to a lower tension in an effort to reduce this failure mode.
The above image is of a
preventive maintenance mod based on that TSB, in which the IC is desoldered, socketed, and then reinstalled with a small heat sink/load spreader on the tab. IIRC, that is one of a number of mods performed by well-known
eBay vendor 2465b on all his "Refurbished/Guaranteed" 24xx scopes.
I'm not exactly certain I believe that is a better approach than simply ensuring good thermal contact with the large copper fill directly under the IC, though all evidence I've seen on the subject indicates Tek deliberately placed these two star washers under the chip which clearly defeats any such heat sinking to that fill, and never reversed that design. My own gut reaction is that if I ever have reason to attempt such a service, I'll put some silicone thermal pads underneath the IC and shim the tab to eliminate any preload stress.
FWIW, none of the 3 2465s I own have that ribbed plastic/rubber/vinyl what-the-fuggever-it-is on the bottom front, though the suggestion that it serves to prevent occlusion of that most important vent over the U800 certainly sounds plausible. Possibly some vendor-added spacer for a 2465 that was assembled into/made part of a larger, special-purpose test gear?
mnem
*tzzt*