spudrow, my 550 1 is one that is mentioned that didn't come with an assigned serial# If you would like to find out how old yours is check out the info below. I have tried to shoot shorts through mine and it will but it does jam at times with the shorts.
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If you will call or email us the serial number and model number we can determine the approximate age of your firearm.

Serial numbers were not required until 1968. For models without a serial number, we may be able to determine the age by the 2-3 letters that are stamped on the barrel.

Contact us through our Help Center by email or call us at 800-243-9700 Mon-Fri 9-5 EST.