Hi George,
If you have the SF616 motor, you install it with the hump...up.
The 3A and, I believe the 3B used the SF616. The easiest way to tell if you have the right motor is the vacuum connection tube has a little curl. A 2A, M38 motor etc...doesn't have the curl. It's straight. My understanding is that the vacuum internals work backwards in a 2A motor so I'm guessing that you would have to mount it up-side-down to get proper function if mounted on a 3A.
Did any of this make sense?

Speaking of that, my lever is broken on my SF616. See Photo. If anyone has an old junker motor that has a good lever, I'm in the market.
Tim