The Tapco 20 round mags are very well done and do work reliably. However if used in a wood stock you must remove some wood from the magwell to insure that the mag does not get squeezed at all. Any pressure on the sides of the mag will almost always result in a failure to feed. It's a simple operation.
If you use the Tapco T6 stock, no modifications to the magwell are necessary because the magwell is already wide enough to accommodate the mags.
The adapter you're talking about is designed to use the same Tapco mags, but there is no such thing as a duckbill-less Tapco 20. You have to remove the duckbill yourself.
There is only one good reason to use this adapter, and that's to make it easier to carry Tapco mags in pouches. The duckbill makes pouch use practically impossible. Without the duckbill it's possible to use a standard AK mag pouch.
If using pouches for your SKS mags is not important to you - then the adapter is meaningless and you better off just using the Tapco 20s as is.
However, a basic dump pouch will carry at least three Tapco 20 mags, with the duckbill, easily and works just fine.
I use a pouch that's something like this, except mine is a bit smaller. It holds three mags comfortably. I'd guess that this one will hold at least one additional mag.
http://www.cheaperthandirt.com/MOLLE175-1.html
As far as 922r, there's confusion regarding parts counts and that's partly due to BATF and partly due to Tapco.
Tapco says the Op Rod is a compliance part and BATF says the SKS doesn't even have an Op Rod.
Tapco says that the T6 stock is three parts, but a compliance part is a part that *replaces* an existing part, since there is no original pistol grip on the SKS, the T6 pistol grip is just *adding* a part and not *replacing* a part. So the T6 is really just two compliance parts rather than the three parts Tapco wants us to believe. There are other examples.
So do your research carefully before deciding to play the 922r game. For me it's worth it. I like the SKS a great deal better than the AK.