I got a Samsung ZM-60 from T-Mobile (incl. SIMlock). The first shock came after switching it on for the first time: Everything was in the T-Mobile magenta color. After some research, I found out, that it's originally a SGH-Z300.
Instructions on how to flash a new firmware are on handy-faq.de. There also was a nice collection of firmware images on anvi.it, but seems to be down for now.
The best firmware seems to be the Z300AIEK1, since it is only slightly branded by TIM (an Italian provider?) and contains everything the original Samsung-Firmwares do.
You can use the supplied cable to flash the phone.
9 key and power it on so that the outer display shows “Download” on red backgroundThe flashing takes about 12 minutes and after that, you have the original Z300 Samsung theme.
To remove the SIMlock, there's a manual at gsmhosting.com. You need the Qualcomm Unlocker and a PC with a COM1: port where you need to short the Pins #2 and #3. Then it's a thing of 20 seconds to get rid of the SIMlock.
The phone accepts .jar files from any server if it sends the content type application/java-archive instead of application/octet-stream. This is easily accomplished by adding a .htaccess file with the line
AddType application/java-archive .jar
to the directory where the .jar files are on your server.
The phone sends the following User-Agent to websites:
SGH-Z300 SHP/VPP/R5 SMB3.1 SMM-MMS/1.2.0 profile/MIDP-2.0