Karl,
You are correct, there is a word written in pencil, which seems to read as 'AUF'. Probably just a note to remember to stick this one in the album.
These albums were common items and were used to remind drafted soldiers of their service time.
If I have to guess at the timeframe, the early nazi-era, around 1935. Uniforms are old style and equipment isn't really up to date. The Velox photo paper isn't much help, as this was in use from the 1920s until the 1980s.
Sadly, many of these albums are being torn apart, as the individual images make more money than complete albums...
Here are some examples of such an album, still in complete untouched shape, dating from 1936.