Tiger Cub Rookie Oil Change Move

Hi all,
First time poster here from New Zealand.

I have a 1968 Tiger Cub (T20 Super Cub), I've had it for a year and love it to bits.

I'm a rookie with bike maintenance and tried to tackle a gear oil change for the first time today. I took the drain plug out and not much drained out so it was well overdue for a top up.
Anyway I put the drain plug back in and took out the overflow bolt in the middle of the drain plug. I poured and poured and nothing came from the overflow but I managed to overflow out the top where I was pouring.
I'm embarrassed to say I think I've got the wrong hole, I also used 20w 50 which I've realised afterwards probably isn't the best choice for the gearbox.
I have a photo of where I poured the oil, any help on what to do would be greatly appreciated.

https://ibb.co/VMMb7hB

 

Thanks heaps for the reply Jim, I'm working on getting a manual. Oh no, I guess if that's the points housing where I have put the oil then I'll have to take the whole cover off now? Not as simple as finding a drain plug to drain I guess?