5900 would likely correctly be a 1973 as most '74s are numbers above 6100. DeTomaso was well known for weird registration sequencing for a host of reasons.
As Larry and Mike have noted, the original Group 4 cars are typically numbered 23XX or 28XX. There may have been one or two outside that range (factory conversions or other), but I don't think 5900 would be an original Gr4 car. The price should be an indicator as well since original Group 4 cars would be priced at 2 to 4 times a "conversion".
Post some photos and details if you can.
Mark
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