Correct spelling for the English word "letted upon" is [lˈɛtɪd əpˌɒn], [lˈɛtɪd əpˌɒn], [l_ˈɛ_t_ɪ_d ə_p_ˌɒ_n] (IPA phonetic alphabet).
The term "letted upon" is not a commonly used phrase in modern English. However, if you are referring to "let upon", it has a different meaning and etymology.
The verb "let" in this context is an archaic form that means to hinder, prevent, or obstruct. It originated from the Old English word "lettan", which had a similar meaning. In Middle English, "let" was commonly used with the preposition "upon" to indicate the act of hindering or restraining someone or something.
For example, if someone says "nothing can let upon my success", it means that nothing can hinder or impede their progress. In this sense, "let" is used as a verb and "upon" acts as a preposition.