Curious as to why left & right holders ?
Er...Um....Well that's what I was shown getting on for 40 years back and it always seemed to make sense.
Consider the bent to the right tool holder in a two slot block carrying a right hand tool for the default towards chuck turning. Block is set-up so the holder is in the slot closest to the work piece and angled so as to put the side of the tool bit parallel to the faceplate. Tail of the tool holder is angled away from the work so it clears the tail stock, or anything in it but not pointing out so far as to interfere with the top or cross slide handles.
Facing tool in the bent to the left holder lives in the other slot facing t'other way. In use the block is swung round to put the side of the tool bit parallel to the lathe axis. As before this puts the tail of the holder into, usually, clear air with reasonable tail stock clearance. Swop the bent to left holder into the other slot for a left hand turning tool.
Inactive tool tip is fairly out of the way, unlike the porcupine with a sore head attitude of a fully loaded 4 way. Can work this way using just two slots of a 4 way but the wider tool post gets in the way rather at times when compared to a purpose made two slot of half the width.
Clearly working this way makes it very easy to template grind any desired leading edge angle or shape onto the tool bit when you know its going to be held either at 90° to the spindle axis or parallel to it.
Won't do for all jobs but I found it a decent standard set-up on Portass S, Pools Special, SouthBend 9" C then A and, finally, Heavy 10. The Smart & Brown 1024 I now drive came with a Dickson tool post and "enough" holders. Over three years in and I'm still unconvinced that its a better system overall. The lathe is, of course, beyond reproach.
Clive
Clive