Convoluted story. The SU-7B had originaly four store pylons, each one of them could carry 500 kg´s/1100 lb, two under the wings and two under the fuselage. Late builds of the SU-7BKL received another two pylons under the wings, to a great total of six, that configuration was retrofitted to older “BKL”´s and quite a few even older “B”´s.

http://www.airvectors.net/avsu17_1.html#m2

Cheers