Actually the first two generations of the airport extreme N wireless model have same model number but different part numbers.
January 9, 20071st generationMA073LL/AA1143802.11a/b/g/n*
August 7, 20072nd generationMB053LL/AA1143802.11a/b/g/n*
You need to check the product ID by checking the number inside the firmware.
This is the listing from version.xml several years old of the first three products in the N wireless airport series.
Double check the Product ID first and if you cannot upgrade firmware I will check if I can help in some way..
- Product ID 104 = (7.6.7, 7.6.4, 7.6.3, 7.6.1, 7.6, 7.5.2, 7.5.1) 802.11n AirPort Extreme Base Station (AEBSn) 1G, Fast Ethernet
- Product ID 105 = (7.6.7, 7.6.4, 7.6.3, 7.6.1, 7.6, 7.5.2, 7.5.1) 802.11n AirPort Extreme Base Station (AEBSn) 2G, Draft-N, Gigabit Ethernet
- Product ID 106 = (7.6.7, 7.6.4, 7.6.3, 7.6.1, 7.6, 7.5.2, 7.5.1) 802.11n Time Capsule (TC) 1G, Draft-N
As you can see A1143 can have product code of 104 or 105 depending on the year.
So the way to get product ID is as follows.
Use airport utility export the configuration.. this is option in the top menu while you have edit open.
In the exported configuration file.. which will be airportname.baseconfig
so in my case tcgen4again.baseconfig
Open the file in text edit (whatever)
Look for the following keys
<key>syAM</key>
<string>TimeCapsule6,116</string>
<key>syAP</key>
<integer>116</integer>
Voila the internal apple designation for your airport.
In a 5.6 airport utility you will use,
One thing I have done is get earlier firmware by going through my Time Machine backups for a version.xml file from a while ago .. and get the link to much earlier firmware.. not all firmware is listed..
I don't have an Extreme that old but here is a Time Capsule. Gen1
This is showing the full summary page.
And the list of firmware does not go back far enough for you.
Frankly the Airport Extreme Gen1 should be placed in the trash.. but up to you.. using a >10 year old router is clearly not going to match modern equipment.
Nor would I over worry about the firmware... that is the least of your issues.. as it is unlikely to actually fix or improve anything.