伍 is an alternative Japanese numeral that is used in formal documents. This documents can include legal documents. When they are used in legal documents, these set of numerals can prevent contracts, checks, and other documents from being changed or manipulated after printing. For example, the standard kanji for 1, 2, and 3 can be incremented up by the addition of a single stroke. Therefore, more complex kanji like 壱 (one) are used to prevent this practice. We encounter the same problem with Hindu-Arabic numerals in English, so we parenthetically spell out the number in English for the same effect. For example, “the merchant agrees to deliver 2 (two) units to the customer.”
There are, however, other uses for these kanji. They have a certain cachet to them. They are old-fashioned and formal, perhaps a little more Chinese than everyday numeric kanji. In the manga you saw it in, it probably is meant to look cool. Being set in the Taisho period may have contributed to this style decision, and furthermore the “coolness factor” is applicable even to stories set in the present or future (e.g. Neon Genesis Evangelion).