missing itemprop on 'product_specific_references mpn'

[Sviz]

Hi, the MPN has not the itemprop markup and Google can't recognize it

I tried to insert into file Theme/templates/catalog/_partials/miniatures/product.tpl 

    {if $product.specific.references.mpn_to_display}<meta itemprop="mpn" content="{$product.specific.references.mpn_to_display}" />{/if}

and into file Theme/templates/catalog/_partials/product-details.tpl
a new block:

    {block name='product_specific_references mpn'} 
        {if isset($product.specific.references.mpn_to_display) && $product.specific.references.mpn_to_display neq ''}
          <div class="attribute-item product-specific-references mpn">
            <label>{l s='Specific References' d='Shop.Theme.Catalog'}</label>
            <span itemprop="mpn">{$product.specific.references.mpn_to_display}</span>
          </div>
        {/if}
      {/block}

but it dosn't work

Any suggestions?

Thanks