Colt ArmaLite AR-15 model 01

Hi guys

This rifle is coming up for auction here in Switzerland. It is a fully functional full auto rifle. I might bid on it and try to buy it if the price is reasonable.

I am not familiar with this model, can someone help me out here, I see a slab side, forward assist, guess it’s an early one. Or maybe the upper is not original to the lower ?

Thanks in advance

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