SBC reverse setup - revisited

[Bill.Light at kp.org]

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Despite contrary views - I DO have reverse authority over my SBC assigned 
IP addresses:

I am using a SuSE distributed linux, 9.2 Professional  with bind 9.2.4-3

I have  x.x.x.120/29 authority, but SBC is telling me that I need to have 
an in-addr-arpa for .120 ??

.121  is the slipstream dsl modem
.122  is my first (usable) IP  that is my firewall   - I do NOT want DNS 
there
.123  is my  Name Server

/etc/named.conf

zone "29/120.x.x.x.in-addr-arpa" in {
        type master;
        file "db.x.x.x.120-29"
};

zone "123.x.x.x.in-addr-arpa" in {
        type master;
        file "db.x.x.x.123"
};

in /var/lib.named

db.x.x.x.123
===usual===
123.x.x.x.in-addr-arpa.  IN NS  blah-blah.
123.x.x.x.in-addr-arpa.  IN PTR  more.blah-blah.

db.x.x.x.120-29
===usual===
29/120.x.x.x.in-addr-arpa.  IN NS  blah-blah.
29/120.x.x.x.in-addr-arpa.  IN PTR  more.blah-blah.

(Note - same "blah-blah" for both)

I could abandon the "120-29  29/120" construct and replace with  just 120
but isn't this then assuming that .120  is my primary ?

Am I missing something obvious ?

Mail is/was working without being flagged "may be forged" which I used to 
get before I changed to this configuration, which dnsreport.com  reports 
as a non-responsive server (and the backups have no authority).

If I need to post full IP/domain, let me know...I'm just being paranoid.