the tank, shrouding, and head design post date when that bike was made by quite a few years. On another note though The more I look at your bike I kinda think it was a tank on engine/small block (HS40) configuration from the factory though, Not the remote tank style like in the brochure you posted.
If there is no indication of tank mounts welded to the frame and if you pull some tape off that seat to find piping along the top edge of the seat separating the heat pleated top from the sides rather than a skinny center pleated section that runs up the center of the seat it was a tank on engine one.
I did find an engine number in my book they used one of those, it was 143.617092 and was an HS40. This thread and good photo was supplied by member smallbore a few years ago:
https://oldminibikes.com/forum/index.php?threads/what-model-is-this.121388/ thankfully its alot cleaner of a shot than all I could ever find (only pic I have on file is posted in that thread) and his bike shown is a solid example of one.