XF 2.1 Best/only way to call a page node id in a conditional?

[frm]

Is this the best or only (accepted) way of calling a page node id in a PAGE_CONTAINER conditional?

<xf:elseif is="$xf.reply.containerKey == 'node-51'" />

I mean, it works, but shouldn't there be something like $page.node_id or something?

Thanks