2400bps

It's the bit rate for a modem. 2400 bits per second. Capable of 240 bytes/sec of download (with overhead).

It was good for is BBSing and faxing.

300 bytes/sec comes from 2400 bits per second / 8 bits per byte is where this came from.

This number is not entirely correct. Each byte of data transmitted by a serial interface also needs a start bit and one or more stop bits.

Therefore, each transmitted byte requires at least 10 bits giving a maximum data throughput of 2400 bits per second / 10 bits ber byte of data which equals 240 bytes of data per second.