I recently had to delete my answer to Why doesn't James Webb have a larger supply of thruster propellant? because @NgPh noted:

As the last update, JWST weights 6200 Kg. jwst.nasa.gov/content/about/faqs/facts.html

Which basically invalidates the argument I was making. I then went searching for other sources to verify this and arrived at a clashing list of stakeholders/sources (as of Dec. 16 ~15:00 UTC):

6,500 kg Camp: 6,200 kg Camp: Other (~6,200 kg):
Northrop Grumman ("main industrial partner") ESA (Last Update: 15 December 2021) JWST Mission Operations Concept Document (Jan. 13, 2006): "The current Observatory mass is 6,194 kg (MR-99)"
Canadian Space Agency (Date modified: 2021-12-15) NASA 1 James Webb Space Telescope Project Mission Requirements Document (Oct. 17, 2007) MR-99: "The JWST Observatory wet mass shall not exceed 6,159 kilograms"

(Note NASA 1&2 are literally the same website)

How much does JWST actually weigh?

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    $\begingroup$ What is the mass of the JWST might be a better question, particularly once it is in orbit... $\endgroup$
    – Jon Custer
    Commented Dec 16, 2021 at 15:27
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    $\begingroup$ To throw in a third value, this paper quotes 6330kg (not clear if wet or dry mass). $\endgroup$ Commented Dec 16, 2021 at 23:34
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    $\begingroup$ The values from 2006 and 2007 can be discarded for two reasons. One is that 6194 kg, 6159 kg and 6200 kg are essentially the same number. The other reason is that those figures are from early on in the construction of the spacecraft. Many things have changed since then. However, mass limit oftentimes is one of the things that remain constant throughout a spacecraft's design and implementation. $\endgroup$ Commented Dec 17, 2021 at 2:08
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    $\begingroup$ I see two possible sources for the discrepancy, and there might well be others, so these are just guesses. Guess number one: The sites that say 6200 kg haven't updated since 2007. Yes, those sites say they've been updated recently, but that's mostly for launch dates. If true, this would not be the first time that NASA or ESA has carried forward numbers that are fifteen years out of date. $\endgroup$ Commented Dec 17, 2021 at 2:15
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    $\begingroup$ @NgPh Mass properties are amongst the most closely carefully tracked and configuration managed of information pertaining to a spacecraft. All analyses involving mass is done through numbers obtained from a configuration managed database or spreadsheet. Unfortunately, NASA (and now apparently ESA) has dumbed down their outward-facing websites to the extent that they are nearly worthless. This dumbing-down of outward-facing websites has been on-going for 20 years. Those outward-facing websites are maintained by public outreach or authorized personnel. Funding is low, and the results are sad. $\endgroup$ Commented Dec 17, 2021 at 11:49


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