TD's book, "An Amateur's Guide for the Colt's Thompson Submachine Gun," mentions the "JHB" stamp several times and specifically on page 173. There are two significant components to the "JHB" stamp that involve shipping dates from AOC and original finish of a Colt TSMG.
"Most, but not all Colt's will have the JHB inspection mark on the upper right side of the receiver. If this mark is struck very light or uneven, review carefully for signs of buffing/reblue."
The prevailing lore regarding retired U.S. Army Major James H. Barrett inspection mark "JHB" is that, as an employee of AOC working at the Colt Hartford plant, he performed this service on finished Colt TSMGs during the little over a year production period before storing them in the warehouse on the Colt factory premises. If this were true, what explanation could there be for him not stamping thousands of Colt's with his "JHB" mark? Is it more likely that Barrett stamped his "JHB" mark on a Colt TSMG at the point of sale?
Of course Barrett's duties consisted of much more than stamping Colt receivers. He was responsible for keeping records of orders to be shipped to customers. He is even mentioned in the FBI case 52-505 investigation into the "shipment of arms and ammunition on S/S Eastside Sinn Fein activities."
To my knowledge, the dates of Barrett's employment with AOC have not been established. But perhaps the date of his departure from AOC can be connected to the time when his "JHB" stamp stopped appearing on thousands of Colt TSMGs.
As the "JHB" mark can be found on low and relatively high serial numbers of the 15,000 Colt TSMGs (Colt Navy #12666 has "JHB" stamp) there seems to be a cut-off point as it relates to the date of sale of a Colt TSMG. Making a deduction from the reported sales and comparing them to examples of Colt's without the "JHB" stamp, that date occurs around June, 1934. That the National Firearms Act was enacted on June 26, 1934 may be coincidental.
Colt #11410, supposedly one of the 951 Colt Thompson's purchased by the US government on June 30, 1939, does not have the JHB stamp (see photo). Are the rest of the 950 missing the stamp as well? Are there any "JHB" stamps on the 3,000 Colt TSMG Model 1921A and Model 1928 ordered by France in 1939 or the January 25, 1940 contract of 500 ordered by the Royal Swedish army? No "JHB" stamp on these examples of Swedish Colt TSMGs M/40. (see photos). Add all of these up with examples of police department Colt TSMGs without the "JHB" stamp and the number seems to close in around 1/3, or more, of the 15,000 Colt TSMGs do not have the "JHB" stamp.
Swedish Colt 1921A no JHB stamp.jpg 37.48K 20 downloads
Swedish Colt TSMG m40. no JHB markjpg.jpg 47.54K 16 downloads
TD's book includes photos of examples of "JHB" stamps that are demonstrative of more than the presence or absence of this stamp. There appears to be evidence that Barrett stamped his "JHB" into finished Colt TSMGs already blued receivers before shipping.
P. 63. "JHB" stamp on Colt #4328 looks blued over.
P63 JHB.JPG 48.68K 14 downloads
P. 49 Colt #4953 "JHB" stamp not blued over
p.49 Colt #4953 JHB not blued over.JPG 39.17K 14 downloads
Colt #6320 not blued JHB stamp (see photo)
JHB unblued inspector stamp on Colt #6320 sold 2010.jpg 9.76K
Colt #9807 not blued JHB stamp (see photo)
Colt #9807 JHB unblued stamp.jpg 18.18K 10 downloads