The above MCM Stereo to Mono likely uses a Transformer to not only convert but to provide electrical isolation between the circuits.
There are ways to do Passive Stereo to Mono Conversion -
Or this -
Scroll down to the second to the last project on this page.
The goal is to isolate the two outputs from each other.
For sake of simplicity and consistency, let's assume we use 680ohm resistors, which is a standard size. In either case, the signal path between the left and right channels has 2x680 = 1360 ohms, therefore the output channels can not short together.
The Input to the Mono Circuit is likely in the range of 10,000 ohms to 50,000 ohms, so the 1k or 2k ohms in the passive summing circuit is insignificant. In the last Circuit with the "X" Pattern, 1360 ohms relative to 10,000 ohms means about 13% of the signal is lost in the circuit. If we apply the same principle to the first circuit above, then 6.8% of the signal is lost in the circuit. However, if the imput impedance of the circuit you are feeding is 47kohms, then in the first "X" Case, 2.8% of the signal is lost, in the second case, 1.45% of the signal is lost. All these are acceptable.
Standard resistor values that could be used in these Passive Circuits would be 470 ohm, 560 ohms, 620 ohm, 680 ohm. You choice as long as they are all the same.