This morning, Jesse and I started work on fitting the new intercom to the plane. However, after a while, we noticed some things in the instructions and the hardware that began to concern us. Like the fact that the supplied headphone jacks were mono, not stereo. And the fact that the wiring diagram showed only 2 wires going to each headphone jack.
Then the truth hit us.
This is a mono intercom...
Before everybody bursts out laughing, let me quote the text from the Aircraft Spruce catalog (Page 494 for those of you who wish to read along) for the intercom:

The ITC-402P ENRI intercom is a small, easy to install intercom designed for the pilot who flies a small 2 or 4-place aircraft. Featuring single volume control operation, the pilot and passenger can enjoy quiet communications on those long cross-country trips. THe ITC-402P includes a stereo auxiliary audio input. To prevent missing ATC communications, the music is muted when either audio from the radio is present or there is intercom audio.
The new ITC-404S-P has full auxiliary stereo in and stereo out, with dual function muting.

Here is an example of the old saying that Assume means something that makes an Ass of U and Me...I read the phrase "includes a stereo auxiliary audio input" as implying that this was a stereo intercom. My reasoning was quite simple; if his was a mono intercom, a mono input would suffice.
This is not what it meant at all. When Jesse and I squinted at the small print on page 2 of the installation guide, this is what suddenly leapt out of the page at us:

Auxiliary Audio Stereo Input
A Stereo Auxiliary Input is provided allowing a tape or CD player to be connected to the ITC-402P. The audio output is mono to the headphones.

"mono to the headphones"....the terrible truth dawned. This is a mono intercom, which was not what I thought I had purchased.

After several minutes of sotto voce use of Anglo-Saxon, we pulled out the Spruce catalog and looked closely at the descriptions again. From reading the last line of the description, it is clear that the 2-place version (the ITC-402P) is a mono intercom, but the 4-place version (the ITC-404S-P) is a stereo intercom. This fits with a pattern I have seen with other manufacturers, where their 2-place versions are mono but the 4-place variants are stereo.
This intercom is going back to Spruce first thing on Monday, and I have ordered an ITC-404S-P. Fortunately we had not actually used any components of the intercom, so I was able to put it back in the box totally unused.
Third time's a charm? We'll see. In the meantime, I learned once again that it always pays to read the small print very carefully...