What you're seeing isn't any kind of "dialect spelling" or "pronunciation spelling", but rather the historical spelling. Historical spellings are the kana spellings from before the spelling reforms of the late 1800s up through the mid-1900s. These older spellings often reflect the etymologies (word roots) of the terms.
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「モノ」「もの」「物」, There is a reason to use for each of them in many situation, but few people use them consciously.
Therefore, there are many people who use them incorrectly.
Some people use hiragana to soften the mood of a sentence.
there are general rule.
モノ： When it includes other meanings as well as the meaning of the kanji
もの： When used as ...