My mistake, D batteries, not C.
I notice that SturmeyArcher22 has not actually said that there is a Dynohub on the bike, but I think we have assumed that there is. Assuming there is one, a good page on the device is
here.
The way the system works is that the battery and the dynamo are alternative power sources for the lamps. Normally you would use the dynamo, and switch over when moving slowly or stopped. In the first variant linked above, there is a three-position switch in the headlamp. With the second, switchover is automatic but at some cost in efficiency and brightness. The semiconductor rectifiers of the time were poor by modern standards. In the first system the dynamo is linked to the headlamp, then on to the rear lamp and batteries. With the second it should be dynamo->battery->headlamp->rear. Either way, the lamps should work with no batteries inserted.
Using LED bulb replacements with a dynamo requires a bit of care. The ones linked above will not work as direct replacements because the dynamo generates AC and the seller states that the bulb is polarised. Almost any bulb replacement requires an additional regulator, because the dynamo wants to make increasing power with speed. For the same reason, only the correct incandescent bulb will work well as it needs to balance the dynamo characteristic. (But almost any low-voltage bulb can be used to verify operation.) The site I linked above has everything needed, at a price. If you know a bit of electrical theory it is easy and cheaper to make up something from generic parts.