In a nutshell
There are three versions of the digit 7 that are in use in Germany, Austria and Switzerland since the 1940’s:
- The upper line is curved and there is a slash in the middle
Taught in schools in Austria from the 1940’s (maybe even before) until today. This versions was also taught in DDR (between 1958 and 1968)
- The upper line is straight, and there is a slash
Also taught in Austria from 1995 until now (as an optional alternative to version 1) and in DDR between 1968 and 1989 and in Switzerland since 1947.
- No slash in the middle, but the upper line is curved
Version from Germany that was introduced in 1941.
Sorry, I don't know which versions was in use in Germany (BRD) after World War II, because I didn't find examples of writings from that country that shows the writing of digits.
The British and American seven (upper line is straight, without middle-slash) never was in use in German spoken countries (and still isn't taught in schools here).
I was born in 1965 in Austria and entered school in 1971. There I learned »Österreichische Schulschrift 1969« ("Austrian school writing 1969"):
This writing is a minor variation of »Österreichische Schulschrift 1947« (for which I didn't find an example to show here)
As you can see, the digit »7« has exactly the form you showed (well, with a much more intense curving at the upper line, which I interpret at the writers personal style)
Since 1995 there is an additional form in use in Austria.:
Since 1995 teachers can decide if they teach the writing from 1969 of the newer version from 1995. Both versions (but no other version) are still taught in schools in Austria.
When looking at the digit »7«, you will see that in Austria this digit has in both versions a slash in the middle, while in German schools the digit »7« is taught differently:
This writing is from 1941 and since then it was used in Germany:
As you can see, the upper line of the digit »7« is curved, but there is no middle-slash.
Sorry, I didn't find examples showing digits for writings taught in BRD after WW II, but here is a writing form 1958, used in DDR:
The 1958-DDR-seven and the 1969-Austria-seven are identical
But 1968 he writing was reformed in DDR, and the digit 7 was changed:
The next example is from Switzerland, 1947:
As you can see: The upper line of the digit 7 is not curved, but there is a middle slash