Schneider-Kreuznach — 240mm f/5.6 convertible plasmat for 8x10 and 5x7, converts to 420mm f/12
The Symmar 240mm f/5.6 is the long member of Schneider-Kreuznach's convertible Symmar range, usable as a standard lens on 8x10 inch cameras and as a portrait lens on 5x7 and 4x5. Like the rest of the convertible line it dates from the 1950s and 1960s, before the Symmar-S replaced it.
It converts to 420mm f/12 with the front cell removed, and carries both sets of markings on the barrel. The design is the family's six-element, four-group symmetrical plasmat, single-coated, in a Compur 2 or Copal 3 shutter. Image circle is approximately 336mm at f/22, enough for 8x10 with limited movement and very generous movement on smaller formats.
Performance at 240mm is respectable stopped down, with the usual single-coated character of lower contrast and more flare than modern multicoated glass. The 420mm conversion is softer still and benefits from a yellow filter and f/32 or smaller.
It is a big lens, so check the shutter size against your lens boards before buying. Verify both cells are original and matched, test the shutter across its full range, and inspect for the haze that commonly settles inside cells of this age.