Multiple URL from Custom post name

You can add a query var and a rewrite rule to handle the extra URL data-

Add the wpd_cf query var to the list of known query vars:

function wpd_add_query_var( $query_vars ){
    $query_vars[] = 'wpd_cf';
    return $query_vars;
}
add_filter( 'query_vars', 'wpd_add_query_var' );

Add a rewrite rule to handle the incoming requests and set the wpd_cf query var. This particular rule will match upper and lower case alphabet plus a hyphen. WordPress has a default rule in place to capture numbers after the post name for multi-page posts, so this will avoid capturing those requests. The first argument should match the URL slug of your custom post type, the second argument should set the key you registered your custom post type under.

function wpd_add_rewrite_rule(){
    add_rewrite_rule(
        'custompostA/([^/]+)/([-A-Za-z]+)/?$',
        'index.php?custompostA=$matches[1]&wpd_cf=$matches[2]',
        'top'
    );
}
add_action( 'init', 'wpd_add_rewrite_rule' );

Then in the template (or a template filter) you can check the value of wpd_cf:

$the_requested_field = get_query_var( 'wpd_cf' );

Don’t forget to flush rewrite rules after making any changes, this can be done by visiting the Permalinks Settings page in admin.