Personally I would take a slightly different approach to the one you outline above. It is your clock, for the time being. You will never replace what was there originally, because whatever spandrel you put on will not be the exact same ones Knibb would have used. If you like the cherub head spandrels, and they are contemporary with the clock so meet your aesthetic, then I would go with them. Your clock could be a year earlier than the Loomes one. It could be closer to the date of the Darken and Harper one. Knibb could have been using both spandrel types interchangably at the time. Who can tell. And you are the one who has to live with the clock over the next few years, unless you are selling it So go with what you like - a previous owner clearly did!