This was taken from Valmet-weapons.com. Wish it was still up!
5.56mm Magnum Research Rifles
MR no model # (appears to be a Model 361 but is not marked) - 5.56mm AR, 18" bbl., bayonet lug on barrel, no bipod or bipod attachment point, M-16 style flash suppressor, has spring loaded firing pin, rifle is stamped Galil above scope mounting notch. Serial # in MR0051x range. (1 rifle reported)
MR 361 - 5.56mm AR, 18" bbl., 1-12" twist, bayonet lug on bbl., bipod lug milled off gas block, M-16 style flash suppressor. Early models had a "straight trigger" as found on Valmets. Serial # in MR00064xrange. (1 rifle reported)
MR 372 - 5.56mm ARM, 18" bbl., 1-12" twist, bayonet lug on bbl., bipod, M-16 style flash suppressor. Serial #s from MR0028xx to MR003xxx. (3 rifles reported)
7.62mm Magnum Research Rifles
MR 331 - 7.62mm AR, 20" bbl., 1-12" twist, bayonet lug on barrel, bipod attachment milled off gas block, M-16 style flash suppressor. Serial# in MR04xxx to MR046xx range. Caliber is marked &ldquo 7.62/.308&rdquo. (2 rifles reported)
MR 332 - 7.62mm ARM, 20" bbl., bayonet lug on bbl., M-16 style flash suppressor. (0 rifles reported)
MR 332 - 7.62mm AR, 16.5" bbl., no bayonet lug, no bipod or attachment, plastic handguard, M-16 style flash suppressor. Serial # in MR069xx range. Appears to have been a "Special Run" of rifles. (1 rifle reported)
It appears that all MR Serial #s are located behind the rear handguard (vertically) on the "left" side of the rifle.
There appears to be transitional variants of Model 331, 332, 361, and 372 rifles during the MR/AA change over period. The Model 331,332, & 361 rifles as imported by AA hold true for the descriptions listed under their MR brethren. The only difference is they contained milled out areas on the receiver where the MR importation marks and serial # prefixes have been removed and the AA data added.
Some Model 372 ARM rifles have been reported with more noticeable differences. They appear to be an AR/ARM combination. Their features are as follows: Model 372, 5.56mm, 18" bbl., 1-12" rifling twist, black plastic handguard, bayonet lug, bipod attachment point w/o bipod, Galil style flash suppressor, and no carrying handle. Additionally, they have a shallow "milled out area" on the left side of the receiver where the MR importation marks and serial # prefixes have been removed and the AA data added. It appears these rifles were originally MR rifles, but were "redone" to have AA markings. The Serial #s are in the AAL3xxx range and are located behind the rear handguard (vertically) on the "left" side of the rifle.
AA 361 - 5.56mm AR (transitional), 18&rdquo bbl., 1-12" twist, bayonet lug on barrel, no bipod, Galil style flash suppressor. Serial # in AAL000xxxto AAL011xx range and located vertically behind the rear handguard. These rifles are not marked Galil and have milled out areas on their receivers. (3 rifles reported)
AA 372 - 5.56mm ARM, 18" bbl., 1-12" twist, bayonet lug, bipod, M-16 style (early) flash suppressor or Galil style (later) flash suppressor. Serial # in AA03xxx to AAL207750x range. Early rifles had bayonet lugs on a ring pinned onto the bbl. and later rifles have them attached to the front end of the gas block. (6 rifles reported)
AA 386 - 5.56mm AR, 16" bbl., 1-12" twist, no bayonet lug, no bipod attachment point, Galil style flash suppressor. Serial # in AAL207765x to AAL209xxxx range. (4 rifles reported)
AA 392 - 5.56mm ARM, 16" bbl., 1-12" twist, no bayonet lug, Galil style flash suppressor. Serial #s in AAL207910x to AAL207947x range. (3 rifles reported)
AA 392S - 5.56mm "ARM Sporter", 16" bbl., 1-7" twist, wood handguard,carry handle, no bayonet lug, no bipod, but has bipod attachment pt., no night sights, threaded muzzle. (1 rifle reported)
AA 397S - 5.56mm "AR Sporter", 16" bbl., 1-7" twist, black plastic handguard, no carry handle, no bayonet lug, no bipod attachment pt., no night sights, threaded muzzle. Serial # in AAL210xxxx to AAL21005xx range. (3 rifles reported)
AA 710S - 5.56mm &ldquoARM Sporter&rdquo, 16" bbl., 1-7" twist, wood thumb-holestock, wood ARM hand guard, no carrying handle, no night sights, threaded muzzle, bipod attachment point. Serial # in AAL 210022x range. (1 rifle reported)
AA 331 - 7.62mm AR (transitional), 20" bbl., 1-12" twist, bayonet lug on barrel, no bipod or bipod attachment point, Galil style flash suppressor. Serial # in AAL049xx range and located vertically behind the rear handguard. These rifles are not marked Galil and have milled out areas on their receivers. (3 rifles reported)
AA 323 - 7.62mm ARM, 18" bbl., 1-12" twist, no bayonet lug, bipod, Galil style flash suppressor, Serial # in AAL2089xxx to AAL2094xxx range. (4 rifles reported)
AA 329 - 7.62mm AR, 18" bbl., no bayonet lug, no bipod attachment, Galil style flash suppressor, Serial # in AAL209094x range. (1 rifle reported)
AA 332 - 7.62mm ARM, 20" bbl., 1-12" twist, bayonet lug on bbl., bipod,Galil style flash suppressor, Serial # in AAL2078xxx range. (2 riflesreported) There is one unique transitional rifle reported with the following characteristics: Serial # in MR076xx range and located vertically behind the rear handguard, not stamped Galil, but imported by Action Arms. Yes, it has the MR prefix. Box and paperworkare dated 1992. The receiver does have milled out areas.
AA 329S - 7.62mm "AR Sporter", 18" bbl., black plastic handguard, thumbhole stock, no bayonet lug, no bipod attachment point, no night sights, threaded muzzle. Serial # in AAL209xxxx range. (2 rifles reported)
GS 386 - 5.56mm AR. Serial # in GS2095xxx. No "S" after Model #. (Only 1 riflereported).
GS 710-S - 5.56mm "ARM Sporter", 16" barrel, 1-7" twist, wood thumb-hole stock, wood ARM hand guard, no carrying handle, no night sights. Serial # in GS2100xxx range. (1 rifle reported) There is also a GS 710 rifle reported without the S stamped after the model number and without the GS Serial # prefix. Serial # in 210020x.
7.62mm Springfield Armory Rifles
GS 329-S - 7.62mm "AR Sporter", 18" barrel, 1-12" twist, wood thumb-hole stock, black plastic hand guard, no night sights. Serial # in GS2099xxx range. (1 rifle reported)