The F and J chassis are the same at the back, where the rails are shallower when seen from the side. The L was similarly profiled but the profiling was further aft, rather than longer. and so different at the rear spring hangar. It is this difference that could identify these chasses, chassises - what's the plural of chassis? and I can't remember what the difference was and I cannot see whether the ones in the photo extend beyond the rear spring hangar.
I think they are F's but perhaps an L owner could confirm.