While it appears that the melting and recasting of the "gold" from the vault by Barney and Willy might have been done with, e.g., wax bars, the series' special effects wizard Jonnie Burke actually used bars made of ice cream! By shooting the scene through special filters, the melted ice cream was made to appear as if it were actually molten gold.
When the mercenary searches the truck at the border to inspect the bibles, the guard tower from the Stalag 13 set of "Hogan's Heroes" can be seen briefly.
The primary rifles seen in this episode are Italian BM-59's
Producer Bruce Geller insisted that special effects specialist Jonnie Burke run sufficient current through the hole in the vault floor to make the heating element actually glow red hot. Burke told Geller that the size of the hole that they were working with could not accommodate sufficient electrical current, using the materials available in the mid-1960s, to actually make the bars glow red. Geller insisted that Burke follow his instructions, and as Burke predicted, the wiring burned out. Burke then made the heating element appear to glow by using reddish-orange paint and filters on the cameras.
The tape from which Phelps gets his instructions for the mission looks very similar to the 8-track cartridges that dominated the car prerecorded audio market in the USA at the time, but it's actually the earlier 4-track format promoted by Earl "madman" Muntz from 1962 to 1964. The most obvious difference is the hole on the right rear corner through which the manila envelope below in the glove box can be seen. In 8-track cartridges, there was a pinch roller in that location, but with the player Phelps is using, the pinch roller comes up out of the deck itself and makes contact with the tape when the start lever is pulled. The 4-track format was getting rare even in the late 1960s when this was produced.