The SVT 40 is a better weapon than often credited. The Finns and Germans snatched them up whenever they could find them. They failed in the sniper role (flyers), and that reputation tended to ooze over to the standard model. They were decent in the hands of reasonably trained troops. Give them to shepherd boys out of Mongolia, and trouble will follow. The primary reason for the termination of production was the time it took to manufacture. You could get several M38s, 91/30s, and a bunch of PPD40s for the cost and time it took to make one.
SVT 38s are pretty rare, and will probably fetch several thousand dollars if you see one. Most will be Finn captures, as the Finns kept them in SVT 38 trim, whereas the Soviets converted them to SVT 40 specs when they went back through the factory arsenaling program. The main 38 features were a weaker stock, and a cleaning rod that was let into the OUTSIDE of the stock for some bizzare reason.
Most SVT 40s floating around in the US are factory refurbs. They can be easily distinguished by their plumb colored bolt and carrier, as well as a weak finish on the stock. Even so, these are still legitimate guns, and probably function better than one that made it's way into the US pre 1968.
The 2 port brake was simply a manufacturing consession done after the SVT 40 program began. This being said, it's difficult to give a date when it occured, as they just pulled parts from parts bins of varous ages, especially during rearsenaling. Another concession was the omission of the optical sight grooves on the receiver after the SVT 40 was withdrawn from sniping service. Why machine them, when they will never need an optic. ALL early to middle production guns have these grooves, and their presence does NOT make it a sniper as gun show guys so often tell you. A sniper version will have a "C" near the date and arsenal mark, and it will have a locking grove on the very top of the rear of the receiver. Thus, while infantry/NCO grade SVTs have the grooves that fit the standard SVT sight mount, they will not have the ability to lock the mount on them. I guess they did the math and figured the locking notch would be easy enough to machine later for the selected models.
The SVT is a must have for any amateur military gun collection. This being said, it's no Garand. The sights are poor. No, scratch that, the sights are as good as any other gun of it's age, EXCEPT for the Garand, which had excellent sights. Even though it's light, it's still very long, and unweildy. IMO, the Garand wins in every category of performance, even with the Enbloc system. Keep in mind that the SVT, even though it has a removable box mag, was still designed to be loaded with chargers (stripper clips), and the mag catch is not one you'd want to be dealing with in combat. For the collector, this doesn't matter, but if you are a Tactical Ted, you should pass. IMO, the main defect I've experinced with mine are the fact that they heat up faster than any rifle I've ever dealt with. A few mags (10 rounds each, keep in mind) and it's boiling cosmoline.
In terms of function, if you are getting doubles with milspec ammo, check your firing pin. It might be broken, or it might be jammed with some 40 year old cosmoline. Do not shoot any east block semi autos with US spec ammo as the primers are too soft. The only exception is the early 1949-50 Soviet SKSs, which had sprung firing pins, an option they removed sometime in the 1950 time frame as it was deemed unnecessary and not worth the cost.
Some controversy surrounds whether or not the FN boys completed and perfected their FN 49 (and by extension the FAL) by examining SVT 38s and 40s taken from Finland during the Winter war. The time frame matches up, and the design is VERY similar. Similar enough that if it were two US companies, a lawsuit would have definately been filed. My personal belief is that it was used in the development of the FN designs, but this is not a shameful act whatsoever. One would be stupid not to study a successful design that was available to you, and that is very similar to your own during the time frame when you are proving your concepts. Thus I believe FN started from scratch, but adopted some learnings from the SVT. They probably kept and perfected some things that the SVT did right, and abandoned some things that the SVT did poorly.
Just my opinion...