Folks used to dealing with domestic-made rifles usually forget about the additional restrictions on imported rifles. The 1968 Gun Control Act (GCA) specified that imported guns had to fit a "sporting purpose test" to be imported. Guns with too many military features were banned from importation.
In 1989, George HW Bush signed a bill modifying this section of the law. It prevented imported guns from being reconfigured into configurations that wouldn't be importable, and further restricted what could be imported. The law went into effect in 1990. Guns imported before 1990 aren't affected by these changes, but guns imported afterwards are.
For example, in order for Norinco to continue to import AKs, they had to modify them to a configuration that was importable under the changes to the law in 1990. They had to remove the pistol grip, folding stock, and bayo lug, and ended up replacing them with thumbhole stocks (which aren't pistol grips under the 89 Import ban, but ARE pistol grips under the 1994 Krime Bill). They called this model the "Modified AK of 1990", or MAK-90.
Anyway, your SKS is only "pre-ban" if you can PROVE that it was imported before 1990. Of course, something like 95% of SKSs were imported after 1990, because they weren't very interesting to anyone until AKs had to be butchered. Unless you have an original bill of sale from before 1990, you have a post(89)ban SKS that can't have both detachable mags and a bayo lug. SKSs must also comply with the 1994 Krime Bill, BTW.
Finding information on when rifles were imported is almost impossible. There were lots of importers (many of which no longer exist), guns weren't imported in serial number sequence, and most importers have only hard-copies of their paperwork buried in storage and won't be interested in looking through it all for you. Of course, BATF has records, but they won't tell you the status of your rifle either.
I'd keep the 10-round mag.
-Troy