app/assets/javascripts/spree/frontend/products/show.js
Spree.ready(function(){
if ($('.assemblies_variant').length) {
$('.assemblies_variant').first().show();
$('[name="variant_id"]').on('click', function(){
$('.assemblies_variant').hide();
$('.assemblies_for_variant_' + $(this).val()).show();
});
}
});