Altera ALTCHIP_ID

Hi Folks,

I'm just playing around with the ALTCHIP_ID block that is available in 
the recent Altera FPGAs. This block returns a unique ID identifying the 
chip. The returned value has 64 bits. For my application I should have 
less than 64 bit but still have a very low chance of collisions between 
the numbers. So I wonder which bits I can leave away safely because they 
will be identical on all devices of the same type.
I checked about 20 MAX10 FPGAs and found that the chip-IDs differ only 
in bits [16:4]. All other bits in my small lot are the same. So it seems 
save to cut away the upper range [63:32]. But this is only an assumption 
and I would like to be more certain about that.

Does anyone know how this unique number is composed by Altera. Are any 
of these bits representing information like vendor or device type which 
would be constant for all devices of a given type and could be safely 
cut away for my application?

Thanks,
Andi