Some comments on the cancellation itself;
This cancellation is a Bisected Circle Type Datestamps (JBC-1 in "Handbook of Japanese Postmarks", 2008 Edition, Japan Philatelic Society). There are numerous variation of this cancellation and this particular sample could be categorized as "delivery number blank".
The JBC-1 cancellation consists of information of the post office (upper part, above the horizontal line) and date/delivery information (lower part). The post office information usually consists of the province name on the first line and post office name in the second line, with several varieties such as 3-line version. This sample has a standard format, and the information is as described in the former posts. The delivery information consists of the year / month (1st line), date (2nd line) and the third line which can either be;
- delivery number, using the "I-Ro-Ha" style order for counting the delivery number. ("i"=first, "Ro"=second, "Ha"=third, "Ni"=fourth, "Ho"=fifth, and so on). Unlike the western cancellation/datestamp, the delivery time was not shown in those days.

- "Den-Shin" (telegraph), showing that the stamp has been used to pay the telegraph fee. Several variations such as "Tori-Atukai" (handling), "Den-Po" (telegraph), and "Jyu-Ryo" (accepted) exist with less information in their usage.

- "Den-Wa" (telephone), showing that the stamp has been used to pay the telephone fee.
[no sample in my collection ...]
- or, blank (as in this particular case)

This "blank" line could be either of postal usage at small post office where there was only one delivery per day (i.e. such that there was no need to identify the delivery number), postal usage for acceptance of packets and registered mail in post office branches, or non-postal usage for miscellaneous transactions such as postal savings, telegraph, postal money etc.
As the Fukawaga post office is a rather large post office, we can exclude the possibility that this stamp (and cancellation) is of postal usage, and it is very likely that it was used as non-postal usage for miscellaneous transactions as described above. It is a nice and clean SON cancellation indeed, and would be a nice contributor to your collection !
Collecting Japanese cancellations is quite tricky as the difficulty depends on the combination of the cancellation type and the stamp. There is very little chance of finding high face value stamp with postal usage cancellations, but also very little chance of finding low face value stamp with non-postal usage cancellation, which makes this hobby really fun !
Regards from Osaka, Japan
unechan