What is the product code for RM-1073 Lumia 640 Windows Phone 8.1 T-Mobile?

[awaismshahzad]

Hey guys! Before I start I just want to thank everyone who even reads this thread. Hopefully we can all benefit from this thread.

Now onto my question. I have a Lumia 640 LTE that I bought prepaid from Newegg a couple months ago when they were very cheap. I wanted to get the developer preview but it wasn't compatible so I found Build 10240 here(forum.xda-developers.com/windows-phone-8/general/installed-leaked-windows-10-mobile-t3173000). Now I want to go back to Windows Phone 8.1 of T-Mobile but i'm not sure how.

My questions is:
1. How exactly do I get back to Lumia 640 LTE T-Mobile Windows Version 8.1? (I don't care what I have to install or how difficult it is...I just want to get back to the old version )
2. Will I still get the OTA update whenever it will be coming?
3. What is the product code (or where to find it) for Lumia 640 LTE T-Mobile so I can get the ROM on NaviFirm+ (which I have used in the past)?
3. What is the product code (or where to find it) for Lumia 640 LTE T-Mobile so I can get the ROM on NaviFirm+ (which I have used in the past)?

Thank you so much for any response or help,
Shah