Congrats on your 903SWC! I had one in the early 2000s, sold it in 2004 to liberate money for other project, but missed it a lot. I bought a '78 SWC last December to replace it, and couldn't be happier.
Originally Posted by Atom
Put the 21mm attachment on instead of the 15mm and it seems to be just about perfect. I don't know where I saw it but i read somewhere the specially made finder used the 15mm attachment. Perhaps the lines were designed so you could see some around the frame lines in the finder. In any case I'm just about honed in.
Just be aware that the lens is gathering considerably more subject area than the stock finder is showing. The standard Hasselblad optical viewfinder shows about 85% of the FoV, so a 21mm finder for a 35mm camera shows the right approximate diagonal AoV to match the SWC finder.
The actual FoV of the SWC lens on 6x6 is approximately equal to the short dimension (vertical) AoV of a 16-17mm lens on 35mm. If you go back to the 15mm objective for the Voigtländer finder, your finder image will match the lens actual FoV more closely.
This is why for precise work (critical close focus and framing), most SWC owners use the SWC groundglass back and a tripod.
An optical finder and zone focus make the SWC so handy for snapping photos of people and other casual scenes where precision and accuracy are irrelevant. ;-)