Replacement for the ATSHA204-TSU-T

I want to create my own board for the imp001.
I see no mention on the imp web site that says it needs the ATSHA204-TSU-T ID chip.
Yet all the break outboards have them. Digi-Key and mouser show them out of stock and obsolete.

What is the replacement for them, and why is this part not described or talked about on your page here:

All the ATSHA204 line became ATSHA204A’s:

See also