The type of range hood insert you need will depend on the style of your wooden range hood.
The S Type inserts are designed to fit into Chimney or Mantle style wooden range hoods. The S models are all ultra compact and require no additional metal liners or modifications for installations. The specific model you require is dependent on the width of your chimney or mantel styled hood.
The T type inserts are designed to fit into Tapered style wooden range hoods. The T models will fit in the space between the wooden hood covers with no additional metal liners or modifications required. The specific model you require is dependent on the width of your tapered style hood.
If your preference is for a shallow mantle design above your oven in your beautiful kitchen, then the M type is appropriate for your hood. The 42M filter and power unit is ultra compact and is able to fit directly into most brands of 42 inch wide shallow mantle wood range hood cabinets with no additional metal liner or modifications required.