Airbag module error codes - trying to understand

[elgyqc]

I have decided to try to resolve the airbag warning light issue in my 2000 boxster base. WIth the original airbag control module I got a long list of errors with the most relevant being:
3 – supply
21 - ignition circuit driver
195 - Control module internal fault - All fault codes over 100 indicate a critical fault and usually require a new control module

Because of the error 197 I tried the module from my 2000 S and got only the error:
21 - ignition circuit driver
So it would seem that this is a valid error.
Next I bought a module off ebay that is from a 2003 Boxster S.
All three of the modules I have are identified as:
AIRBAG SENSOR
996.618.219.01
MRSZ2
There is also what I am sure is a build date (week and year), the original from my base is 48/99, from my S 51/99 and the module I bought is 51/02.

When I installed the new module in the base I got the following errors:
21 - ignition circuit driver (as expected)
but also
23 Belt tensioner driver’s
25 Belt tensioner passenger’s
53 Side airbag Sensor driver
56 Side airbag Sensor passenger

So I tried the new module in my S and got, after a reset:
23 Belt tensioner driver’s
25 Belt tensioner passenger’s

So… assuming that the module from my S is functioning correctly and not finding any errors on my S
Why is the new module producing codes 23 and 25 on the S?
Why is the new module producing codes 23, 25, 53 and 56 on the base when the module from the S does not?

Finally, what can I do to correct the error 21?
My plan at this point is to remove the driver’s seat and check the connections to the belt buckle and to check the steering wheel airbag connection, since from what I have read the problem may be in these connections.

Any help or suggestions appreciated.