Who was the other convicted man? Was he a teen or early twenties in 1969?
I assume you are asking about the man convicted of one of the "seven" Co-Ed murders generally thought to have been the work of Collins?
In 2005, a Michigan man - Gary Leiterman - was convicted of the murder of Jan or Jane Mixer, one of the seven "Co-ed" victims. He had never been connected to Collins or to any of the murder victims until a DNA test done on the victim's clothing still in evidence matched with his DNA, which was on file due to a felony conviction involving drugs.
Leiterman denied abducting and killing Miss Mixer, but was convicted on the strength of the DNA evidence. I believe that he was in his 20's in 1969. He had a car which he used to offer a ride to his victim, and he did own a handgun, thought to have been the murder weapon.
John Collins had a close friend and associate named Andrew Manuel, (alias Richard Diaz alias John Stowtack). Manuel was Collins' partner and room mate. They also had a third room mate who might have been involved in, or had knowledge of some of the murders. On one occasion, three men (one of them identified as Collins) were seen by an eye witness giving victim Joan Schell a ride in a red car just before she was murdered.
Manuel's close association with Collins, and his very suspicious flight from Michigan after Collins' arrest are an indication of his knowledge of, and complicity in the murders. In fact, the two traveled together across country from Michigan to California with a stolen camping trailer. They were both in California when five young girls were murdered, and then they left the trailer in California to return to Michigan together in a silver Oldsmobile.
In their short time back in Michigan, on 23 July 1969, 18 year old Karen Sue Beineman was abducted and murdered. Collins was soon arrested and charged with her murder.
Immediately, Manuel left Michigan for California (probably to remove or destroy evidence in the trailer), and then made his way to Phoenix, AZ where he was caught in early August 1969.
During his flight through California, another still unidentifed girl was murdered near Lompoc in a similar manner to some of the Michigan murders. She was stabbed numerous times with a knife. Perhaps just a Big coincidence...
That girl, sometimes referred to as "Lompoc Jane Doe" remains unidentified. Author Sue Grafton based her crime novel "Q is for Quarry" on her case.
I have no idea where either Leiterman or Manuel were on 31 October 1969, but it would be a good question to ask.