SL8 Magazines (30RD) without doing modifications
I don´t know if they will fit the SL8-1, but they fitting the SL8 (german version) very well and are made by H&K.
I don't think it will fit, looks like a double stack mag. Since there were no pre-94 SL-8 mags, you can't import it into this country except as LEO only. 87 Euro- $81, yikes!
The US imported version(SL8-1) will only accept the ten round magazines. The European version(SL8) has a slightly different magazine well and will accept the 30 round G36 mags.