Okay, I freely admit I'm no AK expert, maybe just a tad more educated in them than the average EBR owner but that's probably pushing it.

Regardless, I bought a Vector UF in October '05 and on my particular one, the underfolding stock angles downward when extended.  I have seen pictures of other underfolders that extend parallel to the bore rather than in a downward angle.

When I shoulder mine, I get a normal cheek weld with the sights in line as is.  Nothing feels awkward as compared to shooting any other non-EBR rifle I've fired.  I have a fixed stock Vector and get the same normal cheek weld when I use the Kobra, but to use the irons I have to modify the positioning of my head to lower my eye to get it in line with the sights.  I would assume that the straight line underfolders would need a similar head position as my fixed stock AK.

Since an underfolder isn't going to have the optics rail on the side, why would the stocks ever be set in the parallel with bore plane?  This just doesn't make much sense to me.  Anyone know the answer?

This really depends on what kind of underfolding stock you have and what receiver.  On the stamped receiver AKMS's, such as the Polish kit builds or the FEG Hungarian underfolders, you'll find the underfolding stock to go almost straight out when unfolded.  The underfolding stock has some rivets positioned along both sides of the rails.  Sometimes they're not exactly straight across and some rifles may have a slightly more downward angle but noticebly downward.    

Stamped and milled receiver Chinese underfolding stocks and some milled Bulgarian variants are usually smooth down the rail but will have more of a pronounced downward angle.

If your rifle is stamped but built using a milled receiver underfolding buttstock, that may explain why it's angled as such.  But without photos or knowing the build, it's hard to say.

I've seen some underfolders that were perfectly straight and some downward-angled, all Yugo (Century M70) or Romanian (Century WASR) on the same shelf in the gun store.  All from the same order, and all built from the same parts kits by the same people.  I think some just turn out angled and some don't.

I like the looks of the straight ones better, but now I wonder if I should hold one before buying to be sure I can sight with it comfortably given its particular stock angle.

AK-47 folders have a downward angle (and thick front sight)

vs.

AKM folders are straight (with thin front sight)

I prefer my folder to be straight out. The downward angle seems like it accentuates recoil jump and the stock hits my jaw on recoil. The straight one, on the other hand, sends the recoil straight back into the shoulder and to me it seems less, as well as my cheek/jaw sliding ALONG the folder instead of INTO it.

YMMV.

AK-47 folders have a downward angle (and thick front sight) vs. AKM folders are straight (with thin front sight)

So based on this statement, my stamped AK underfolder has a milled underfolding stock?

^ It is possible.....or, you got a Chinese Type Folder (they borrowed the 47 pattern).

I have been wondeing if a yugo underfolder stock will fit into a chicom trunion or vice-versa.

So what have I got here?  Vector rifle, bought used in '05.  Appears to be Hungarian parts as there is no country symbol on it.  Is this a milled underfolder mechanism on a stamped receiver?

Rommie stamped AKMs use the angle downed folder.

Quoted: i13.photobucket.com/albums/a251/MikeSlisher/DSCN3313.jpg i13.photobucket.com/albums/a251/MikeSlisher/DSCN3314.jpg So what have I got here?  Vector rifle, bought used in '05.  Appears to be Hungarian parts as there is no country symbol on it.  Is this a milled underfolder mechanism on a stamped receiver?

Most likely a Romanian rear end for a stamped receiver.   Front end is hard to tell, FSB look like a milled pattern