Nope. Graduated bore P Lip is a requirement to meet the System design that Charles Peterson patented. Otherwise it's just a reverse calabash.
I would go further to say that without a chimney or "tubular extension" as it was described in the original patents, that sits in the reservoir below the draft hole that a system isn't a true functioning system IE there is no cyclonic actions of smoke moving about in the reservoir causing the wicking of moisture from the smoke before traveling through the stem. I own a system spigot but it doesn't function the same as my Deluxe and premier systems with the chimneys. I consider system spigot half systems or improved reverse calabashes.
Any peterson system smoker will know what I'm talking about, even those modern acrylic stem deluxe systems don't work as well unless you put a little effort into sanding and buffing the tenon so there is less of a wear gap ensuring the chimney sits in the reservoir below the draft hole.
A system reservoir should have moisture in it when you're finished smoking and should require a cleaning, unless you're smoking your tobacco very, very dry.
The actual system pipe patent can be viewed here, pay attention to mention of the tubular extension
He later improved the design with the P Lip.
And of course this in depth blog post from Mark Irwin.
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