Archaisms are such fun! Although "inst." meaning "this month" is still sometimes seen, its siblings "ult." and "prox." seem to be extinct or almost so. From [1] on Alison Kilpatrick's site Arborealis:

• Instant, the current month: as On the 10th Inst. or inst.
• Ultimo, the month just past: as On the 10th ult.
• Proximo, the next month: as On the 10th prox.

"Instant," for instante mense, in the current month; "Proximo," for proximo mense, in the next month; "Utlimo," for ultimo mense, in the last month (Latin).

Source: Brewer, Ebenezer Cobham. Etymology and Pronouncing Dictionary of Difficult Words. London: Ward, Lock, & Co.; New York; 1882.

(cf Ai Ren (2004-03-02), Factor and Factotum (2004-11-04), ...) - ^z - 2019-03-09