MS, AN, NAS, and NASM Part Numbers
A part number stamped MS, AN, NAS, or NASM identifies which standard defines it — the published specification that fixes its dimensions, material, and finish. Placing the prefix is the first step to identifying a part from a drawing, a torn label, or an old assembly, before anything else about the number is read.
MS — Military Standard
Military Standard part numbers are hardware built to a specification published by the Department of Defense. MS is the identification system most items in the federal supply catalog carry today — an MS number on a drawing calls out exactly one part, buildable by any qualified manufacturer to the same published dimensions and material.
AN — Air Force-Navy
AN (historically Army-Navy) numbers are an older joint-service identification system that predates the broad adoption of MS. Many AN-numbered parts are still active and still manufactured today, sometimes alongside a later MS designation for the same or a closely related item — an AN number on an older drawing does not by itself mean the part is obsolete.
NAS — National Aerospace Standard
NAS numbers identify standards maintained by the aerospace industry rather than published directly by the Department of Defense. They cover fasteners and hardware — often to closer tolerances or in configurations — that the MS system doesn't otherwise define, and turn up frequently on airframe-specific drawings and bills of materials.
NASM — a NAS standard carried into the military system
Some NAS-numbered standards have also been assigned a companion military designation; a part carrying the NASM prefix references that same underlying specification under its military-adopted number. Functionally, an NASM designation and its NAS counterpart identify the same governing standard.
The dash number is not a size code you read at a glance
Whichever prefix a part carries, the digits after the dash identify one specific variant — typically diameter, length, thread class, material, or finish — defined in that standard's own dimensional table. That information isn't encoded in a formula readable from the digits themselves; confirming it means consulting the published standard or the part's resolved federal record, not decoding the dash number by eye.
Resolving any of the four to an NSN
MS, AN, NAS, and NASM numbers all resolve the same way: to a National Stock Number carrying the item's technical characteristics, manufacturer cross-references, and supersession history where it applies. Working from the NSN — rather than matching a part by physical appearance — is the reliable way to confirm exactly which variant a drawing calls for. See How to Read a National Stock Number for how that identifier itself is built.