This is Violentmonkey script which will show you product MPN next to its name on Alza.

Alza.cz MPN

// ==UserScript==
// @name         Alza.cz MPN
// @namespace    https://gist.github.com/KristianP26/46cf9a12a8db1dc2127b6db26b9f570a
// @version      1.0
// @description  Extract MPN and Add New H1
// @author       Kristian Partl
// @match        https://www.alza.cz/*
// @match        https://www.alza.sk/*
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    window.addEventListener('load', function() {
        var scripts = document.querySelectorAll('script[type="application/ld+json"]');

        scripts.forEach(function(script) {
            try {
                var jsonData = JSON.parse(script.textContent);

                if (jsonData["@type"] === "Product") {
                    var mpn = jsonData.mpn;

                    var h1 = document.querySelector('h1');

                    if (h1 && mpn) {
                        var br = document.createElement('br');

                        var newH1 = document.createElement('h1');
                        newH1.textContent = `MPN: ${mpn}`;

                        h1.parentNode.insertBefore(br, h1.nextSibling);
                        h1.parentNode.insertBefore(newH1, br.nextSibling);
                    }
                }
            } catch (e) {
                console.log('Error parsing JSON:', e);
            }
        });
    });
})();