had about 7 of them, very stable, had purchased battery from ebay, times cheaper comparing to unhuman digikey. most my units with unknown cal. time. those who where first 198x-year, had a large shift. last one almost dead on ... . it have a few variations of board, i think 7 revisions, where first was 2 colors HP logo and case assembled in 2 pieces, last version, just black logo, one piece of case, and no coils inside for a signal transfer. LCD also has different fonts shape, that one was dead on ... - maybe cal. was 10y ago, and it was a reason of precise measurement.
power filter caps usually became bad, an indirect indicator: short on 30mv produced 10-30 microvolt noise. with a new one: 0.2-0.4mkV
with 110V i don't bother even for main filer. When it calibrated drifting i noticed , maybe 2 last digit on voltage, but much more for resistance (10-15 digits) in 3 years period.