The original immersions with the externally mounted hi limit (safety) stat only had a control thermostat as was the case no doubt with this installation, the last immersion I changed had this (now) combined control and hi limit (safety) stat so no requirement for two safety stats, one of my relations HW cylinder started boiling over recently, I found the external hi limit stat which had only been glued to the immersion boss had become detached and wasn't in contact with the immersion boss, I replaced it with the combined unit.
Good description here from JW of how both stats work, in case anyone is in any doubt that there isn't a separate set of contacts. A electric shower thermal cut out (TCO) use two of these discs (its called a duo disc), one, set to 48C, is self resetting, the other, set to 75C, does not reset, the TCO must/should be replaced, you can see one of the discs below, the other is behind it but I don't know if there are two separate sets of contacts.
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