This week his elder sister Ann Margaret Allan (34), of Falkirk, made the latest in a series of desperate pleas for any information that could throw even the faintest ray of light on what became of him.
The 29-year-old, known as Sammy, was well known around Bonnybridge and Banknock when his late mum still lived in the area, and many there still remember him.
But neither locally nor anywhere else in Scotland has a single reliable sighting been made since autumn 2016, and his family - having all but given up hoping to hear any good news - are as anguished now as when it became clear he really had disappeared.
Ann Margaret said: “We found out he is just one of two cases out of 2518 that remain unsolved. It is impossible to explain to people the agony we have gone through simply not knowing what happened and where he went. We all feel it, all the time - it has put dad in a very dark place.
“Although it sounds crazy you actually want to hear that knock on the door that everyone in this situation should dread, the news that tells you he has been found - and not in a good way”.
She says the police have made a series of major attempts to get to the bottom of the mystery, but that while they’d follow up any clue that might still appear, the case has gone cold.
However, she, along with family members including her younger sister and brother, are not about to give up.
They aim to retrace Sammy’s steps in Glasgow, and find some of the people he seemed to be with at the time.
They will hand out leaflets in the hope that somebody’s memory - or it could be conscience - will finally bring a breakthrough.
“We know he was with a group of people in North Lanarkshire in the Motherwell-Hamilton area at one time”, said Ann Margaret, “yet none of them seem to have come forward with any useful information”. She adds: “For a long time we have feared the worst, because anyone who was in his position would need cash, and yet he had a post office book with several hundred pounds in it that has never been touched”.
While there is always hope, she has long since accepted there is unlikely to be a happy resolution to the mystery.
But because the family are unable to find any sort of “closure” their misery continues, week after week, with no end in sight. His dad has spoken frankly in the past about what he fears must be the truth - if Sammy had been in trouble he is certain he’d have phoned.
However, while foul play is always a possibility in the case of someone who has been missing for so long police, who have repeatedly scoured the Clyde for evidence, can only rate it as a hypothesis.
For a while Ann Margaret thought it possible he may have gone abroad to start a new life, but it didn’t make any sense - if he had done so he would surely have taken the cash he was able to access.
Press accounts from around the central belt last year paint a picture of a young man who may have had his problems, but these were hardly unique.
Young adults go missing all the time, and for all sorts of reasons, and in some cases take care to ensure they are not found. He had no ties to any particular place when he went missing, apparently travelling around the central belt to areas where he had connections - including Falkirk.
Nor has he been known as a conspicuous individual - at 5ft 3ins with stocky build and short brown hair he could easily slot into a Scottish crowd without being noticed.
However, Ann Margaret believes it possible, even likely, that somebody knows - or at least has an inkling - of what happened.
Sammy wasn’t reported missing until the start of 2017, when the family began to become seriously alarmed, but once it became clear his disappearance was a real mystery their attention focused on events around that last definite sighting.
Ann Margaret says she feels some decisive event could have happened immediately after October 19, 2016, and that one or other of the two most likely scenarios - accident or foul play - happened then. She is determined he will not be forgotten, and wholly committed to the effort to find the truth.
But at the same time she hopes somebody who, for whatever reason, has so far kept information to themselves will at last come forward - and give her, her dad and siblings the chance to find some kind of peace.
Back in January last year police said of Sammy’s disappearance: “Enquiries are underway to try and locate Mr Townsley.
“It is not unusual for him to go off by himself for weeks at a time and not maintain contact with his family or friends.
“He has no fixed address and is known to travel to travel to Renfrewshire, Dunbartonshire, Lanarkshire, Glasgow City Centre and the Grangemouth and Falkirk areas.”
Mr William Townsley, Samuel’s dad, said at that time: “Although it’s not unusual for Sammy to not be in touch for a while, he has never left it this long.
“He’s always given us a phone to let us know that all is well.
“I know that he does have a somewhat chaotic lifestyle but that doesn’t take away from the fact that he’s my son and along with his family, we are very worried about him.
“We’ve been in contact with other friends and family to see if he’s been in contact with them and no one appears to have seen him.
“Sammy does travel around quite a bit and I’m hoping that someone, somewhere, has seen him or heard from him.
“Please get in touch with the police - we just want to know that he’s safe.
Ann Margaret’s main fear is that the length of time that has passed with no inkling of fresh information may lead people to assume the mystery just cannot be solved - and she finds that impossible to accept.
She says from her point of view simply giving up isn’t an option, and the search for the truth must continue.