I took a look at the manual for the Model 1, which I found at hifiengine.com. As I interpret the information provided on page 5 of the six page manual it appears that XLR shorting plugs are only necessary if the amp is being used in bridged mono mode and is being provided with a single-ended input. I assume you are not using the amp in bridged mono mode, since you mentioned getting "a" Model 1, which I presume means one amp.
If I am correct in assuming that you are using the amp in stereo mode and providing it with single-ended inputs, the RCA shorting plugs you have in place are all that is necessary.
But if you were to operate the amp in bridged mono mode while providing a single-ended signal to its right channel non-inverting RCA input (as specified in the manual) I’m pretty certain that the "XLR shorting strap" you referred to would be suitable, and in that situation (and only in that **exact** situation) it would be connected between pins 1 and 3 of the right channel XLR connector.
Regards,
-- Al