Yeah agree looks as if 3 years is official from what you posted but it's no problem if the bike isn't road taxed for more years and that is why think some DLT's are different.
When I got my now bike and asked locally about non payment of road tax it was said 10 years and you have to pay for it to be re-registered.
If OP wants the bike in his name if he can contact the previous owner he only needs copy of passport picture page and a copy of the passport page showing the Thailand exit stamp.
Owner to date sign both copies.