You can find the ER-301 board revision number printed on the back near the OUT1 and OUT2 jacks.

Revision 7

Units sold from October 2016 to May 2018 are revision 7. This is the first publicly available hardware release.

Revision 8

Never released.

Revision 9

Never released.

Revision 10

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.