Serial Number format: H55116328 (This serial number is from actual air-conditioning unit). Date of Manufacture: Using the first and second digits of the serial number H5 5116328 you will find the year of manufacture. The first digit (Always the letter “H” for the 1980’s) is the decade of manufacture Aug 31, 2017 · Your 9U series D6 is a 1951 model wide gauge. All Cat track type machines year of manufacture from 1925 to 1960 are available in a book form from the Antique Caterpillar Machine Owners Club (ACMOC) online store for about $20.00. Book is called Track-Type Machines, 1925 to 1960 Serial Number Reference Manual. Glock Model 26, 9mm caliber, serial numbers DGU, DHR DKU. Glock Model 27, .40S&W caliber, serial numbers DGD, DGV, DHS, DHT, DKV, DKW, DKX. Mr. Don Bulver, warranty department of Glock, Inc., indicated the recoil springs produced before September 1999 may shear off on some of these pistols due to over hardness of the support tube.