To add to what Arthur said, the first numbers are the part number followed by the transmission number in the form 722 xxx xx xx xxxx.
This link might help: http://www.altousa.com/mersmod2.htm.
It says you should have a 722.400 just as Arthur said.
If it's any consolation, my 81 SD came with a 722.309 transmission (early 80s 280E). I replaced it with a 722.315 (early 80s 300Dt). These are obviously not the correct transmissions but they were modified or rebuilt to work with my car.
Assuming all physical dimensions and connections are identical (check the speedo drive in particular), that 123 prefix might not be an issue. At least note the numbers from the transmission in your car and insist on a match when you go shopping. Note that the last 4-8 digits of the transmission number might be a unique/unmatchable serial number.
What 6 bolts are you talking about?