Batch Script Adding a Period (.) to File Name

Hi, I am testing a script to find the last file within a folder.

It can locate the correct file, but the resulting variable includes a period (.) within the file name.

Here's the script:

REM Copy the recent Friday evening's Diff Backup onto the drive G: My Passport device.
@echo on
set srcDir=F:\Backup\Differential Backup
set destdir=G:\My Passport\Differential Backup
cd /d F:\Backup\Differential Backup
FOR /f %f IN ('DIR /b /od *.zip') DO @SET last=%f
echo The most recently created file is %last%
:::
echo "%last"
:::
:eof
pause

This is the Windows Command results:

Microsoft Windows [Version 10.0.19045.3448]
(c) Microsoft Corporation. All rights reserved.
C:\Users\Todd>@echo on
C:\Users\Todd>set srcDir=F:\Backup\Differential Backup
C:\Users\Todd>set destdir=G:\My Passport\Differential Backup
C:\Users\Todd>cd /d F:\Backup\Differential Backup
F:\Backup\Differential Backup>FOR /f %f IN ('DIR /b /od *.zip') DO @SET last=%f
F:\Backup\Differential Backup>echo The most recently created file is %last%
The most recently created file is Diff-.56_S.zip

You will notice the actual file "Diff-56_S.zip" appears in the variable %last% with a period(.) immediately after "Diff-".

I have tried various script versions, but every one results with the period (.) withing the variable.

What causes that to happen?

Thanks!