A more likely source for the gun was Slack.
Hanratty admitted asking Slack to get a gun (I “asked him to get a shooter to do some stick-ups”) but later claimed “it was only a matter of conversation”. From prison he wrote a letter to Slack setting out this as the explanation.
Had Slack been allowed to receive the letter he would have been able to concur that ‘it was only a matter of conversation’ and support Hanratty’s line of defence. Instead he denied the discussion had happened at all.