You can find the ER-301 board revision number printed on the back near the OUT1 and OUT2 jacks.
Units sold from October 2016 to May 2018 are revision 7. This is the first publicly available hardware release.
Units sold from June 2018 and onwards are revision 10. The differences are quite minor:
- I removed the 5V Source Switch to prevent damage to the ER-301 hardware via the 5V bus line when using the BUS setting. The result is that the ER-301 no longer uses the 5V bus line.
- I added an I2C header to allow integration with the Monome Teletype without having to perform any hardware modifications.