Service Tag information lost in BIOS when motherboard replaced

ASSET 209   Asset Version 2.09

Release Title: System Utilities:
Dell Asset Utility, Utility, English, Multi System, 2.09

Release Date: 09/27/2005
Description: Asset Utility

ftp://ftp.us.dell.com/utility/R109071.exe

Utility : Dell Asset Utility
Version 2.09
For DC051, DXP051, DXC051, DM051, DE051, DXP061, DXC061, DM061, DV051; Dimension : 8300 / 8300N, 2400, 4700, XPS Gen 3, 8400, 4700C, 5000, 3000, 9100, XPS (Gen 5), XPS Gen 4, 5100C, 5100; OptiPlex : SX280, GX630, GX530, GX620, GX520, 210L, 170L, GX280, 160L, SX270 / SX270N, GX60, GX270 / GX270N; Precision : 360 / 360N, 670, 470, 370, 380, WS390, 690, 490
On Not Applicable
Languages : English

Usage:
ASSET new_asset_tag Specify new asset tag
ASSET /d Delete asset tag
ASSET /o new_owner_tag Specify new owner tag (if applicable)
ASSET /o /d Delete owner tag (if applicable)
ASSET /? This help screen

There is an undocumented command switch which you can use with asset.com. It
is the '/s' switch, and works exactly like the '/a' and '/o' switches.

Thus, the command:

ASSET /s XXXXXXX

Will create the Service Tag represented by the X's.

I dont know what my service tag is and its not on my dell Optiplex GX520 anywhere. I bought it used and replaced the Mobo. How can I create a new one or get another?

Are you saying your service tag and express service code is some kind of numeric equation you can calculate if you know the serial number? What is the calculation if so?  My case has a serial number, but no sticker with the service tag or express service code and its not found in my bios. Is there anyway to recover it? I bought the machine used.