Strongroom (1962) *solved*

[walden]

Black & white film: late ‘50s early ‘60s. Two young men hold up bank: lock staff in the strong room. Later, the men realize that the supply of air will be insufficient and that the trapped people may asphyxiate. They call police and return to bank to help to force open safe doors. The doors are opened and the two men are walking up the stairs under arrest when someone calls out that one of the staff has died.

[DB7]

Strongroom (1962)

[walden]

Yes, that's it! Funny how films seen in one's teens stick in the mind. Thanks!