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{if isset($product.reference_to_display) && $product.reference_to_display neq ''}
      <div class="product-reference">
        <label class="label">{l s='Reference' d='Shop.Theme.Catalog'} </label>
        <span itemprop="sku">{$product.reference_to_display}</span>
      </div>
    {/if}

Insert this code after line 125 for example but from the product.tpl file

the file in this path: themes> name theme> templates> catalog> _partials>product.tpl

Save and then delete the same code from the other file product-details.tpl if you do not want it to be displayed on the product sheet as it had previously

PS: Always keep a copy of the original file without modifying, for possible errors or mistakes, so you always have backup