Modify SKU for display on product page

Try using the core WooCommerce filter woocommerce_get_sku:

function wpse_188691_woocommerce_get_sku( $sku, $product ) {
    if ( ! is_admin() ) {
        $parts = explode( '.', $sku );

        // Only first two parts
        $parts  = array_slice( $parts, 0, 2 );

        // OR all parts except last (remove above)
        // array_pop( $parts );

        $sku = implode( '.', $parts );
    }

    return $sku;
}

add_filter( 'woocommerce_get_sku', 'wpse_188691_woocommerce_get_sku', 10, 2 );