Apologies for the delay in replying.
Both dash1259 and KMerritt refer to <<*.123 files>> and <<.123 format>>.
I find this confusing. When 123 saves a file, the default extension is '*.wk?' where '?' is replaced by the version number of 123.
The following is a copy of the 123 Help file
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Naming a 1-2-3 File
Every file in a directory has a unique name. When you create a new 1-2-3 file in memory through File New, 1-2-3 assigns a default name to the file. The first default file name is FILE0001.WK4; the next default file name is FILE0002.WK4, and so on. You can save the file on disk with this default name or with another name that you create.
A file name can contain up to eight characters. You can use any combination of letters, numbers, _ (underscores), and - (hyphens), and you can use uppercase and lowercase letters (they are equivalent in file names). Do not use spaces in a file names. Instead, use _ (underscores); for example, MY_WORK.WK4.
File extensions
When 1-2-3 creates a file, it adds a three-character extension to the file name to identify the file type:
Extension File Type
.AL3 Named page settings; a file created when you save the current page settings as named page settings with File Page Setup Save.
.BAK Backup version of a worksheet file (.WK4, .WK3, and .WK1); created when you save a 1-2-3 file as a backup file with File Save As.
.FMB Backup version of a 1-2-3 format file (.FM3 and .FMT); created when you save a .FM3 or .FMT file as a backup file with File Save As.
.MAC Macro for a customized icon; a file created when you customize an icon and save it with an associated macro using Tools SmartIcons Edit Icon.
.NS4 1-2-3 shared file; saved on a Lotus Notes server.
.TXT Text; created when you save a file as a text file with File Save As.
.WK1 Worksheet; created when you save a file with File Save As as a 1-2-3 for DOS Release 2 worksheet file.
.WK3 Worksheet; created when you save a file with File Save As as a 1-2-3 for Windows Release 1 worksheet file, or 1-2-3 for DOS Release 3 worksheet file.
.WK4 Worksheet; created when you save a file as a 1-2-3 Release 4 or Release 5 for Windows worksheet file with File Save or File Save As.
.WT4 Worksheet; created when you use File Save As to save a file as a SmartMaster template.
.XLW Excel workbook file; created using File Save As.
.XLT Excel worksheet file; created using File Save As.
.DBF dBASE IV file; created when you use File Save As to save a selected range as a dBASE IV file.
.DB Paradox file; created when you use File Save As to save a selected range as a Paradox file.
You can assign another extension to a file by specifying the extension when you save the file. The extension must begin with a . (period) and contain up to three characters.
For example, you can save a 1-2-3 Release 5 (.WK4) worksheet file as a file with the extension .WK3 (1-2-3 for DOS Release 3 and 1-2-3 for Windows Release 1 worksheet files) or .WK1 (1-2-3 for DOS Release 2 worksheet files) by using File Save As and changing the extension in the file name.
Related SmartIcons
Saves a file
See also
File Commands
Named Page Settings
Sharing Files Using Lotus Notes
Specifying a File
Working with .WK1 and .WK3 Files in 1-2-3 for Windows
Working with dBASE and Paradox files in 1-2-3
Working with Excel Files in 1-2-3
Working with SmartMaster Files
Working with Text Files in 1-2-3
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If a user specifically saves a 123 file with the extension '*.123' (e.g., 'expenses.123'), this will be why the file cannot be read by Excel.
HTH
Richard