I have a Zeiss Protar VII D set in barrel (29/35/41/48cm). The Protar cell threads match a Compound #3 shutter I've got (the filter ring from the D set fits the shutter), but the tube length on the shutter isn't long enough to accommodate the cells. Does anyone know the designation of Compound #3 with long tube (as seen with Protar D sets in shutter)? Or can someone confirm if Zeiss Protar cells have the same threads as Compound #3 B&L Protar VII cells? I've found that the cost of getting extension tubes machined isn't much less than buying a B&L Protar VII in Compound #3 shutter.