A used M-65 purchase should not be settled by the size on the tag alone. The most useful checks are the contract label or stock number, the zipper material, whether the jacket is shell-only or liner-equipped, and the actual garment measurements listed flat. Those details matter because M-65 sizing is not simply a modern Small, Medium, Large system, and the removable liner changes how the jacket fits.
Start with the labelled size, then verify with measurements
One sizing guide describes M-65 jackets as typically sized by chest measurement and height rather than only standard alpha sizes. In that guide, a Medium Regular might fit someone with a chest size of roughly 37 to 41 inches and a height of 67 to 71 inches. A Medium Regular label therefore represents a chest and height range, not just an alpha size.
Retail M-65 size charts usually give garment measurements, not body measurements, and the common format is inches taken laid flat. Many charts measure the chest from armpit to armpit, often called pit-to-pit. Some also list shoulder, back length, and shoulder-to-cuff. One retail chart, for example, lists XS at 20 inches pit-to-pit and Small at 22 inches pit-to-pit. A vintage listing may show a Large Regular with a 26-inch armpit-to-armpit measurement and still instruct buyers to check all measurements below the tag size.
Shoulder width matters as much as chest because the M-65 has a boxy military cut. One guide advises not relying on standard S/M/L labels and instead measuring the chest at its widest point and shoulder width from seam to seam.
Look for the contract label and stock information
Most M-65 jackets have a contract label inside the collar. Earlier models can also carry a contract label on the lining under the bottom right pocket, plus an instruction label in the same area. If a listing does not show that label, ask the seller for a close-up before deciding.
The label can carry specific identifiers. One 1975 large regular example includes the contract number DSA 100-75-C-1200 and stock number 8415-00-782-2942. Those numbers identify that specific item, not every M-65, so they should be read as a reference for what a complete listing photo may show rather than a universal date code.
Use the zipper material as a dating clue
Zipper material can help date an M-65. Chromed alloy zippers were used from the first contract in 1965 until 1971, brass zippers were used from 1972 until 1986, and plastic zippers were used from 1986 onward. The zipper type is only one identification clue, but it can be compared with the contract information in the label when asking the seller for better photos or clarification.
Account for the removable liner before you buy
For surplus M-65s, the removable liner changes how you think about size. A jacket that fits comfortably as an outer layer may feel much tighter when the liner is buttoned in. Check whether the listing is for a shell only, a complete jacket with liner, or a liner-equipped configuration that is not fully photographed.
Layering intention should be part of the sizing decision. One guide advises sizing down one full size if you plan to wear the jacket only over t-shirts or thin shirts, while staying with your true size if you intend to layer it over a hoodie. Because the M-65 is historically cut for layering over sweaters and gear, the intended number of layers can change which chest and shoulder measurements actually work.
What to confirm before paying
Before buying a used surplus M-65, confirm that the listing gives you the tag size and laid-flat measurements, a clear label photo with contract or stock details, zipper material, and liner status. The practical takeaway is to compare the contract label, zipper material, liner configuration, and pit-to-pit measurement together rather than trusting a military size label alone.