Why are do some addresses on the Monero testnet begin with '9' and others with 'A'?

Question

I thought the prefix of a cryptonote address was fixed. For example AEON uses a Wm. And Monero a 4. But sometimes the Monero testnet has addresses that begin with a 9 and sometimes they begin with an A. Why is this?

Example - these are both valid XMR testnet addresses: 9uXRFi4PZMqhsnthBF6bGdfVnBSZtfKkR7Td8qPM7jUKZeTfR1tLhCoTLqYNE12xuiQg3aWGiLw83bWsqwTRLaM4Jk47xYM

A1kVSWahxwRM2sABJzDWj5ZX1eJZ2FzuWLPTU3Kru97wQYNMrqex6k91LGALK3qvnMNVBPHEY76YqjWG4SB3y9SVR6tcypR

Answer 1

Prefix is fixed in that the first (AKA network) byte is fixed to 0x35 for testnet. When encoding to base58 to get the actual address, the first 2 characters will vary depending on what follows after the network byte.

Those 2 addresses decode to:

353e39ae5e839ff8f46141a37551e6bae627575a6d8da8be269dae6f3410e188c32d446e4e687c749d7d165250f740b58d885ec452f317d8ce5a35de40ef08679d428d4a16

and

35e280bafa4b88c077c3b05ad67691acc268b5feb93c02e973e88a7a78b86a228cbdcf062b8776a201fc46bdd3fc1bda80749a241d0df4eafe1e1f17bb067ec6d5a0213a66

so you can see they both start with 0x35, which is followed by the spend public key, view public key and the checksum.