Before the 1970s, there weren't really any guidelines to use for assessing bail amounts. It was a subjective matter of whether or not they thought the person would show up in court if they were released from custody. In 1974 Congress created a 10-point system that interviewers could use to identify whether someone was a good fit for a PR bond (pre-trial release bond, where the defendant pays nothing, also called ROR or "released on own recognizance") and to guide judges on making bail assessments. Before that, though, it was just up to the judge.

Bail amounts are still set by the arraigning judge, so it's up to their discretion, without going into the "unreasonable amount" territory. (You can't set a $100,00 bail on a criminal trespass, for example.) In 1966, the Bail Reform Act allowed defendants to pay 10% of the total to secure their release rather than the total amount (same as today). $6 in 1968 is roughly $45 today, so if we were looking at a modern offense that could incur a $450-ish bond, then you're looking a low-level offenses like possession under 2 ounces, assault with bodily injury (for a first offense), unlawful carrying of a weapon, etc. (I used to work in a jail booking department, so these are the first few that came to mind.) Assault with bodily injury could, and often does, refer to things like fist fights, and I wouldn't be surprised if the songwriter had something similar in mind when writing the lyric.