Your 1981 C70 does not have a 6 Volt, or even a 12 Volt ignition coil.
The 1981 C70 is a 6 Volt bike, however it features an AC magneto ignition coil. This means that AC power generated in a stator winding that is separate from the lighting and charging system, is used to power the ignition coil, points and condenser.
An ignition coil from a CT70 would work just fine, such as this one on the Dr. ATV website:
It even has the correct Black/White wire and two bolt bracket.
1970-73 C70 Honda Service Manual: http://tinyurl.com/6ebwtw
1970-73 C70 Troubleshooting Guide:
http://tinyurl.com/6ebwtw (scroll to sec.7.1 page 101.jpg)
1980-83 C70 Honda Service Manual: http://tinyurl.com/hu42c
1980-81 C70 Troubleshooting Guide:
http://tinyurl.com/hu42c (scroll to sec.18-1 page 170.jpg)
1982-83 C70 Troubleshooting Guide:
http://tinyurl.com/hu42c (scroll to sec. 20-28 page 205.jpg)
1980-81 C70 Maintenance Schedule: http://tinyurl.com/z4zn6
1982-83 C70 Maintenance Schedule: http://tinyurl.com/hw35c
More resources for C70 Passports and Cubs: