If it is a 486 DX the co-processor is built in to the chip. If it is a 486 SX there was no co-processor built in to sell them cheaper. If your friend's is a 486 SX I doubt whether you will be able to get one separately.
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Synology invites you to visit them at Insomnia on 26th – 29th August. Book your tickets now and visit Synology.