Hi, Tim.
The answer is simple.
The parts list/diagram doesn't always show the actual shape/location of each parts for all Year models.
As you experienced, there is only one diagram on the parts list for the two different shape/location of Main relays for DBW and non-DBW models.
Presuming that you have RHD model and if you want to be sure, please have a look at under the bonnet or inside the engine bay.
DBW model
You need Main Relay #39400-SL0-A01
You will see LOOPED black cable surrounding the brake fluid reservoir and connected to the black plastic cube box inside the front compartment under the bonnet.
Also, at the right side of the engine bay, you will see thick black hose with red text on it. This is the fuel line from the fuel filter.
On DBW model, you WON'T see thick cable running in parallel to the fuel line like the one on non-DBW model below.
non-DBW model
You need Main Relay #39400-SL0-003
There is no looped black cable surrounding the brake fluid reservoir but still, you will see black plastic cube box nearby so don't get confused.
At the right side of the engine bay, you will see thick black hose with red text on it. This is the fuel line from the fuel filter.
On non-DBW model, you WILL see thick cable running in parallel to the fuel line. This is the TH cable running all the way from the front to the TH body at the left side of the engine and hence, non-DriveByWire.
I'm not a fan of Year model as sometimes, people get confused with registered year against Year model.
You may have 97 registered NSX but not necessarily 97 Year model NSX.
The best is to check against the VIN but if you check the above photos and 100% sure that your NSX is DBW, then your Main Relay is located around the right shoulder of your driver seat for RHD model.
Wow, more than GBP109 for the Main Relay.....
I'm quite sure if you ask Andy at vtecdirect, he would be able to source it much cheaper.
I always keep one each in stock at my place for owners.
Depending on the exchange rate and delivery charge from Japan/US, I would assume it would be about GBP50.00.
Regards,
Kaz