Hello, in the boot there is the same function as in the firmware to identify the pattern of PCB. I think memory is a memory 0x12009000 own the pcb that identifies it.
When you access USB-boot mode, this routine displays the gps as eTREX20, since it is not patched. In normal power mode is shown as eTREX30, (Windows Explorer, Basecam, etc ...).
I pass the link of boot 2.80.[Only registered and activated users can see links. ]
Very interesting TXT file commands, will try to prove if I understand.

Boot: 0418491C sub_4598491C; CODE XREF: +16 sub_459849A2? P
Boot: 0418491C LDR R0, = 0x12009000
Boot: 0418491E PUSH {R4, LR}
Boot: 04184920 LDR R3, = 0x428003C
Boot: 04184922 LDR R2, [R0, # 0x20]
Boot: 04184924 MOVS R1, 0x1E0000
Boot: 04184928 Orrs R2, R1
Boot: 0418492A STR R2, [R0, # 0x20]
Boot: 0418492C LDR R2, [R0, # 0x24]
Boot: 0418492E BICS R2, R1
Boot: 04184930 STR R2, [R0, # 0x24]
Boot: 04184932 LDR R0, [R0]
Boot: 04184934 LSLS R0, R0, # 0xB
Boot: 04184936 LSRs R0, R0, # 0x1C
Boot: 04184938 STRB R0, [R3]
Boot: 0418493A LDRB R0, [R3]
Boot: 0418493C STRB R0, [R3, # 1]
Boot: 0418493E BL sub_4599380A
Boot: 04184942 POP {R4, PC}
Boot: 04184942; End of function sub_4598491C