There is more to your model number than you have given. There should be 5 or 6 digits after the period. If it is the model I think it is there is nothing in the way of a keypad. There is a key switch that can be hardwired to the unit but the problem is every crook and his brother know how to get into your garage if you have one of these. Do your remotes still work? Please post back.

There are a number of after market rolling code remotes available. Hooking the receiver up to the opener is very easy, and then a new code is used every time the door is opened. The old dip switch remotes only had 256 codes (8 switches) and were never really very secure.

The owners manual is no longer available and the odds of finding someone that has one is very high. What is the problem with the opener?

Hope I'm not just "talking to myself". I went to HD and purchased the Chamberlain Model 955D at $50+tax. It appears to be an exact replacement for my (10 year old, thereabouts) Craftsman 139.53683. I also discovered in this process the "Smart" program button, it is under the front pop cover, no need to remove the three screws that give one access to the internals of the receiver. Well, this gave me a chance to check the old units using the new units and I was surprised to find that both the receiver and transmitter appear to be bad. The old remote will not program to operate the new receiver, and the new remote will not program to operate the old receiver: conclusion both the old remote and receiver failed at the same time. This seems highly unlikely, maybe the old and new are not interchangeable.