You typically pay more for a Long Telephoto (105mm and higher) lens in 39mm Leica mount than their SLR counterparts. The optics are usually identical. Be careful of buying a Soviet SLR M39 mount lens: finding an adapter might be difficult.
The FSU 135/4 comes in M39 RF coupled mount for the rangefinder camera, and 39mm Zenit mount for the SLR. The adapter required for each is different.