They could be screwed together or just bonded with epoxy. As was said cheaper on boxes/packing materials and also shipping. Also when John called me about this project and he said the original was cut I was kind of glad. The USDM and JDM bumpers aren't the same physical size and this lets the USDM bumpers use the JDM lip as well as the extra takes up for sagging etc.
Also if you look close you'll see that one side is stepped back behind the other for thsi reason. When I install mine I plan on cutting out an aluminum doubler from 1/8 inch to place above the urethane of the JDM bumper with rivnuts installed so I can drill through the lip, bumper then aluminum and when there drilled install the rivnuts and then use machine screws to secure this extra tight before paint. Having a piece of aluminum will also assure a flat level foundation for everything. For those of you thats never heard of a rivnut run by harbor freight.
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they also have 100 pack of replacements for 10 bucks each.
How they work the aluminum rivnut or nutsert is screwed on to the puller. You stick this in the correct size drilled hole and then squeeze like a rivet but it doesn't pop off. You then unthread the puller and you have a perfectly threaded hole in a piece of thin aluminum. There's thousands of these on every aircraft in the air right now. The first set of aviation ones I got were hundreds I almost had a heart attack when I saw these at harbor freight and bought every package they had. Sorry these are all standard threaded ones no metric. /ubbthreads/images/graemlins/banghead.gif