BattleMech Manual (p. 88)
An Obsolete ’Mech is one built using technological and/or construction standards that have been completely abandoned, such as any Primitive ’Mech from the Star League era until the Jihad. Though still serviceable—and maybe even effective—an obsolete ’Mech is extremely difficult to maintain after its parts sources go silent, forcing technicians to resort to jury-rigging and hand-machining even the most basic structural parts and electronics to keep it battle worthy.
A ’Mech with the Obsolete quirk cannot have any variants in active production, and must note the year in which its production ceased as part of the quirk. After that, the ’Mech will become increasingly hard to maintain, and its resale price will drop (for anyone except antique collectors, that is). To reflect this, for every 15 years after production of an obsolete ’Mech ceased, any rolls to repair or replace equipment on the ’Mech—or to even locate said parts—will suffer a +1 Target Number increase (to a maximum of +5). In addition, for every 20 years after production of an obsolete ’Mech ceased, the resale cost (but not the purchase cost) will decrease by 10 percent (to a minimum of 50 percent).
A ’Mech that somehow comes back into production after becoming obsolete automatically negates this quirk 10 years after returning to production. Additionally, some ’Mechs may have parts manufactured for them even if the chassis themselves are out of production, and thus would not qualify for this quirk. Note that an obsolete ’Mech may still receive additional modifiers for other quirks such as Difficult to Maintain, Easy to Maintain, Non-Standard Parts, and Ubiquitous.