Depending on who made it and when it might have a kovar leadframe, or even a screen printed set of traces. The leadframe will be covered with the glass frit till almost at the window, but there will be some voids down the centre. Probably best to use the OTP plastic, as that will have the kovar leadframe only, and you just grind till you touch the end of the leadframe by the pins. Do not go further as you will cut the thin parts of the leadframe and kill the chip.