卜 (복)
...is the Hanja for 'fortune telling' or 'divination'.
From https://kuiwon.wordpress.com/2016/04/12/why-do-voting-stamps-in-korean-polling-booths-use-the-character-%E5%8D%9C/:
The first elections in South Korea were held in 1948 under the
auspices of the US Army Military Government. Voters at polling booths
used a circular stamp or sign (“○”) with no circumscribed shaped
inside to mark down their candidates of choice. Resources were so
inadequate that people resorted to using the round edges of pen caps
to indicate their votes. The Korean War from 1950 to 1953 devastated
the country even further. In the elections immediately after war,
voters used bamboo branches and even bullet casings to mark their
votes. While Korea’s economy vastly improved in the following decades,
the plain circular mark continued to be used.
The plain circular mark, however, had a few practical problems. When
they are cast, the ballots on which the mark is recorded are folded.
In many instances, this caused in the dye being transposed onto the
contacting side, thereby resulting in invalid votes. In 1992, the
Chinese character 人(인) for “person” was added and circumscribed into
the circular voting stamp to remedy this problem. The addition of this
character shape, however, did not completely resolve the issue with
invalid votes, since the character 人 is somewhat symmetric. The dye
transposed onto the folded side of the ballot was still
indistinguishable from from the side where the mark was originally
stamp.
During the presidential elections of 1994, another issue arose. The
character 人 was seen as too similar to the si-ot ㅅ in candidate Kim
Young-sam’s (金泳三, 김영삼, 1927-2015) name and ultimately viewed as
favoring him. (Kim Young-sam would indeed later go onto win the
presidential election, becoming the first civilian to hold the Korean
presidential office in three decades.) To address this problem, the
character 人 was changed to 卜(복). This alteration also finally
resolved the issue of invalid votes resulting from the dye being
transposed onto the other side of the ballot. Since the character 卜 is
asymmetric, election talliers would be able to distinguish the
transposed mark from the original mark.
The character 卜 also carries multiple meanings, highly pertinent to
the rite of voting. It can mean “to foretell” (점치다), “to consider in
detail” (상고하다), or “to count” (헤아리다). And this is why the voting
stamps in Korean elections have the character 卜.