Checkout fieldsets in code

I’m doing some custom code where I want to access WCRGSK checkout fieldsets. In particular, there’s a function in the WooCommerce Software Add On called order_complete where I was using $order->billing_serial to successful get a serial number that customers enter for calculation of a serial code for my products. However WCRGSK requires checkout fieldsets in order for fields to be conditional. My question is how to I access a custom fieldset created with your plugin in code. I tried $order->extra_serial but that didn’t work. This was with the assumptions that my custom fieldset was named extra and the extra field under it was called serial.

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