The letter is plainly built on sha. Whether the element below it is the letter tverdo, making the letter a Cyrillic ligature, or whether the whole sign was taken from the Glagolitic letter for the same cluster and redrawn to match sha, is argued. Both accounts are current and the shape is compatible with either.
What would resolve this A close comparison of the earliest attested forms of the letter across the recensions, looking for a period or region in which the two elements are still drawn separately, which would settle the ligature reading, or for early forms too remote from sha and tverdo to be a ligature at all.
OpenSlavic palaeographyDivinity Atlas Open Questions Register, First Edition