After some research I have found on some cheap phones it is a *#xxx# code similar to the *#06# code to read the IMEI.
for example:
http://hackaday.com/2017/05/11/teari...omment-3767262

FWIW while I was in school in the US I got the cracked Motorola software(ran under WINE) in the early 2000s to, with a serial cable, SIM unlock and edit the IMEI among other functions on most period Moto phones.
I reset several phones to zero out the IMEI as an experiment (I wrote down the real IMEI in case) just to see if they would associate with network and they fully functioned for several years until I upgraded.
Those who live where the edit is illegal will read and not do, but those who legally are permitted will share an advantage that is for now reserved to those who have bought the unlock cable to process blocked stolen phones for illicit resale.

That part is irrelevant though. The OP wanted to clone one N900 into another. Same make and model. The operator will never notice.

Not that I would recommend it though. Or see any legitimate reason for even trying.

I have broken so many electronic(and mechanical) gadgets trying to hack on extra functionality, I might have been sad but I am not sorry that I had to learn something new. Mostly it is a principlet stand that my N900 belongs to me, I like to know how to access all of it's functions before it expires from natural entrophy, even if I never plan to use them.
It is like picking locks for fun, illegal to use on your neighbors house without permission, maybe it is a suspicious ability to people who do not know why I learned this skill and teach it to others including my kids, but still legal and fun to have the skills to make and use the tools on my own locks.

I present the below page as an example which shows the screenshots of one gray/black market phone kiosk unlocking device which permits changing the IMEI and lists the N900(N9 too) as one phone it can hack. I believe it is a JTAG cable. A hack is in the wild but it is exclusively for those who receive stolen phones, not for us.
https://www.infinity-box.com/
Criminals already have their hack, but not individual owners of N900s.

You may notice that it lists them under Chinese-Nokia
I would not give much weight to that listing.

Either way, good luck. Please report something if you find it. It would be mildly interesting.