To clarify some misinformation. You cannot purchase the LRBHO from either company. If you are wanting the factory LRBHO, get in line as it hasn't been available to the US market for sometime. Rusmilitary carried it only once and even then it's a project and a half to get it installed. I am assuming what Magicmonger is referring to is the BHO which is a different cat in itself.
You will need to modify your receiver to install the magwell properly. Yes, this requires taking material away. There is a template floating around the forum, find it before you start as it has all the correct measurements. You will also need to modify for the front lug as well. Tools you will need are a dremel, some files, and a caliper. Again, don't start this project without the template in hand and get the thought of the LRBHO out of your head. The chances of this happening are slim to none.
For whatever reason, if you back out of the project PM me and I'll purchase the parts. No I didn't post the above information to scare you away and take your parts. Just making you fully aware of what you are proposing.
Edited: For spelling errors
I appreciate your reply. My reference is the VEPR 12 which I do have. I am assuming the S12 has a similar design, if not identical. I am almost complete with the parts I need to make the -030, including the AK-100 stock assembly with pad, I am missing the LRBHO parts and the S12 front latch assembly.