More than 30 student veterans found a carte blanche from the U.S. Treasury in their mail boxes yesterday, but the pass-key to Fort Knox turned out to be only an illusion.

One of the automatic machines at the Boston treasury office was the cause of the trouble. It started missing a cylinder last week, and before the difficulty was detected it had issued 500 GI subsistence vouchers completely made out except for the amount due.

Cause Confusion

A few of the blank checks arrived here yesterday, and most of the puzzled recipients promptly appealed to the Office of the Counsellor for Veterans. There they were told to go to Room 710 of the Post Office building in Boston, where all the machines are in good condition, and the missing amount will be typed in by the proper authorities.

John U. Monro '34, Counsellor for Veterans, cautioned last night that anyone who receives one of the blank checks should not attempt to fill in the amount--even if he thinks he knows what it is--since any alteration of the sum would entail a heavy penalty from Uncle Sam.

The temperamental robot will not affect the delivery of the rest of the October subsistence checks, which are being mailed out on schedule, Monro added.