"signature bytes xrefed against boards"
Boards can be made with any chip. Easy enough to browse the datasheets and collect the signature bytes.
Here's a starting point:
3 Signature Bytes
ATmega48A 0x1E 0x92 0x05
ATmega48PA 0x1E 0x92 0x0A
ATmega88A 0x1E 0x93 0x0A
ATmega88PA 0x1E 0x93 0x0F
ATmega168A 0x1E 0x94 0x06
ATmega168PA 0x1E 0x94 0x0B
ATmega328 0x1E 0x95 0x14
ATmega328P 0x1E 0x95 0x0F
3 Signature Byte, JTAG Part Number, JTAG Manufacture ID
ATmega164PA 0x1E 0x94 0x0A, 940A 0x1F
ATmega324PA 0x1E 0x95 0x11, 9511 0x1F
ATmega644PA 0x1E 0x96 0x0A, 960A 0x1F
ATmega1284P 0x1E 0x97 0x05, 9705 0x1F
ATmega640 0x1E 0x96 0x08, 9608 0x1F
ATmega1280 0x1E 0x97 0x03, 9703 0x1F
ATmega1281 0x1E 0x97 0x04, 9704 0x1F
ATmega2560 0x1E 0x98 0x01, 9801 0x1F
ATmega2561 0x1E 0x98 0x02, 9802 0x1F
Suppose we could summarize all this and put it in a Playground page.