This is an alternative form of the character 五【ご】 ("five"), as you correctly surmised. The entry at the English Wiktionary currently only lists this as used in Chinese, but the corresponding entry at the Japanese Wiktionary lists this under the more general heading of 「漢字【かんじ】」, and describes this as 『「五【ご】」の俗字【ぞくじ】。』 ("informal variant for 五【ご】").
I think this form might have arisen from handwriting. I've seen images of 五 where the writer starts the top stroke, but doesn't draw the line all the way across, and after a very short horizontal, they start the downward vertical stroke. In certain cases this can look like the top stroke has been mostly omitted.