After making his personnel selections for the mission, Jim closes the clasp on the dossier folder. The close-up on the folder shows the clasp open.
When Jim is reviewing the video tape of the bank vault safety deposit box Belzig is holding, there is a big black 127 on the side, yet a moment later he then strains to peer through a tiny telescope magnifier to view the much smaller 127 on the front of the box to confirm the number.
The brick wall in the tunnel entrance uses mortar to secure the bricks. Rollin with the assistance of Paul the bank robber erected it that day. When a few hours later Belzig pounds on the wall it is solid yet mortar takes at least 24-48 hours to set so it should have been possible to knock the bricks loose and escape.
Phelps wouldn't be able to read the box number on the video footage, even with a magnifier. 1960s video tape wouldn't have the resolution necessary to do so, especially given the size of the number and the distance from the camera.
As Alfred Belzig attempts to abscond with his ill-gotten funds through the vault door in the final scene, he finds a brick wall which was erected by Rollin; viewed in one scene toting actual bricks into the passageway. As he frantically pounds on the wall, the entire wall can be observed moving as one piece; indicating a fake prop of some sort.
The record will self destruct when it reaches the final groove. Since there is only one groove on the side of the record, it would have self destructed immediately.
It is mentioned in the mission tape scene that Belzig appropriated over three million dollars from his victims. But it is somewhat implausible that sum of currency could fit into what appears to be a medium to large safe deposit box.
A lot of effort is made to read the number on Belzig's vault box from videotape, but it would have more efficient to just ID the box's location (second row, second from the left) and have Barney check the number while in the vault.