No they are not all the same though all sell similar parts. Ninety percent of the market is going to sell you a 1969-72 Chevelle single piston full floating caliper grabbing an eleven inch disc and selling you a fabricated and drilled caliper mounts to get it to work with your steering knuckle.
Now if you go to your corner discounted automotive parts store and ask them for a Chevy part for your car they are going to hand you a part that was probably made in the bad part of Asia (you know Malaysia, Pakistan, India, or China). The quality is suspect to garbage most of the time.
Living in New Zeeland I am sure you are familiar with oriental policies concerning national industries such as the Japanese government charging their citizens a 100% of the value of that car on any car held more than two years by their citizens to encourage new car sales. Where do all those three year old Japanese cars go? Just look out side; your streets are flooded with them (as is Australia). China claims to have separate companies, but the capital is all fronted by the central government. If the manager (who is encouraged to cut corners on production costs or loose his job, because he has to kick some of that capital back into government minders pockets) messes up and kills a few hundred thousand people they just don't fire him he is taken outside and executed.
You can buy the same parts made in Canada, or Mexico for not much more than the oriental imports (not real sure about the tariffs on goods from the wrong side of the pond imposed by your government, but as I recall they were pretty high favoring Japan and China). Even Japanese manufactured parts are far superior in quality to anything coming out of China.
Here I insist in buying Moog, Eaton, Timken, Raybestos, Monroe parts made in the US (all where GM OEM's), but I have to pay a premium and ask for the part by brand and part number from my NAPA jobber because of the Wal-Mart mentality that drives the parts purchases here and around the world today.
Back in 1961 GM actually made all of their own parts in house and shipped them on trains that they owned on their own tracks. GM at that time made and owned everything. By 1963 the first OEM parts appeared when AC-Delco stopped making their own plugs (they bought Ford Autolite plugs and rebranded them) By 1965 the individual companies that made up GM where being sold off and the money lining the boards pockets (those assets had been paid for by Uncle Sam during WWII procuring war material). So no more GM diesel-electric locomotives or track. Raybestos was the original supplier to GM of brake parts.
Your problem will be in separating the wheat from the chaff as you have to have everything imported. Basically it boils down to get what you pay for, so a cheap price implies low quality. They are all copying GM's design the only thing different is the quality of the parts they used.