PGP : what to do when a user ID has multiple RSA keys

[savron@world-net.sct.fr]

My DOS version of PGP 2.6.2 came with a built in PUBRING.PGP 
containing two keys ( 1024 and 512 size ) with the same user ID .
For testing  purposes I wanted to encrypt a message with the 512 
length key , but only the 1024 size key could be used .

How can I select the right key , apart from making a clone 
PUBRING.PGP with only the key I want to use ?

Any comment welcome