UrS6 Radiator Drain plug AKA petcock

[Jay M]

First of all, who the heck came up and named the drain plug a petcock? I am in the process of flushing out my cooling system of my UrS6 to get rid of all green Prestone PO put in and replace it with G12. I have filled & drained the system several times using only distilled water and all the old antifreeze is out and the water draining out now is as clear as it is going to get. The water coming out now has a slight tint of brown that I attribute to a bit of rust in the original heater core.
I am getting ready to put in a new expansion tank, replace most of the hoses and fill with 50/50 mix of G12 and distilled water but now I see the drain plug (petcock) on my radiator has a very slow dripping leak...about 1 drop every 3 seconds. The petcock is screwed on very tight. I am fairly certain the radiator is original to the car and I assume original OEM radiators were made by LAENGER.+REICH. So I went to my local Audi dealer and ordered a new drain plug petcock for the L&R radiator(part number 4A0121140) There was also a drain plug part number listed on Vagcat 4A0121141 for a Valeo raditor. I picked up the new drain plug and it just doesn't look right. My original drain plug still in the radiator is blue plastic as is this new one I bought but the original also has a couple plastic washers on the threaded end of the blue screw head. More importantly, the slow leak appeared to be dripping from the small blue rectangular shaped piece of plastic located about an inch away (towards the inside of the car) from the end cap of the drain plug. I assumed this little rectangular piece of plastic was part of the drain plug itself but I can't verify because the old drain plug is loosened all the way out but will not come out of the radiator. The new drain plug I bought does not have this rectangular piece of plastic on it.
 
My questions are:
1) Is Laenger & Reich the original radiator that came with my car? Did I buy the correct drain plug/petcock?
2) Regarding the small blue plastic rectangular piece mentioned above. Is that a part of the radiator itself or is it incorporated and part of the correct drain plug that I need to get...part number 4A0121141??
3) In order to completely remove the old drain plug from the radiator do I have to either push in or pull out that blue rectangular plastic piece or do I just use a lot more force with a screw driver and loosen the old drain plug out more and remove. I did not want to initially do that because I am not sure if I would break the plastic. Thanks.