A very nice little addition Ben, congratulations!
Over here these are 13 to the dozen, as they´re pretty much what every kid got from a cheap uncle while growing up...much like i imagine the Jensen 60 is in the states.
That is not said to diminish their value in any way, as they have a a huge "fan base" over here, since almost everyone has some sort of nostalgic connection to this specific model...myself included.
Hard to date within an exact year, but there´s a couple of obvious points, which narrows it down slightly.
This type of TÜV/GS tag was introduced in the mid-70´s, and since it says West Germany, it was put on there before the wall went down in Berlin in 1989.
So your engine was made at some point inbetween those years.
When it comes to the screwed in steamline, the D5 was decommisioned in Europe in the mid-70´s as well, due to safety regulations demanding a sightglass, but was kept in production into the 90´s for export.
I assume that Wilesco then, since these already was offered as kits as well, decided that all the "blind boiler" D5s was made with the same parts, to keep the production of the boilers as simple as possible.