There is an L shaped rod with a black handle on the control panel. Rotate that rod to engage the dog clutch into the drive gear, You may have to rock the tiller back and forth until it engages.

As far as the 2 oil fill holes, one is all you need, (I use the one opposite the carb, it's easier to use,) just drop the depth bar all the way down to level the tiller and fill the oil until it runs out.

According to the SN# provided that Tiller was built 1st half of 1981.

I have a '78 Pony II, an '81 Horse II and an Econo-Horse I don't know the year yet. You have a Pony II.

Good luck!!

The plug on top of the Transmission is for Gear Oil. I Use 85W-140, but TB calls for 140 GL2 oil. On the left side of the Trans, (left side standing at the controls) about half way down is another square plug remove this and if no oil runs out remove the top one and fill with 140 weight oil until in runs out the side.

I have provided the link for the Owners and Parts Manuals. Type in Pony for the model and use either 1 or your serial number in the other spot.

Here is a link that might be useful: Troy Bilt Manuals