Here’s a quote from HWS Vol. III, Chapter 6, which will explain the T275E1 designation. There are three more full pages of information on the International and Palma Match cartridges that follow.
"The Ordnance Committee, on April 9, 1956, assigned the test number T275 to a 7.62mm ball International Match cartridge being developed by Frankford Arsenal. This round was shown on Frankford Arsenal drawing FB 38605 dated April 12, 1956 (Fig. 234).
The T275 used a special 7.62mm match case loaded with a 172-gr. boattail bullet without a cannelure. The primer was the FA No. 26 and the charge was IMR propellant sufficient to impart a velocity of 2,250 to 2,300 fps. The bullet was not crimped in the mouth of the case, and cartridge overall length was 2.880 in. The initial loading was at Frankford Arsenal during April 1956 as Lot FA X7.62-2449 (5,000 rounds) which used cases with [NATO symbol] FA 56 headstamps with a special blue primer sealant. This lot was made for the U.S. Army Rifle Team at Fort Benning and had a figure of merit of 2.5 in. at 300 meters.
An experimental lot of international match cartridges was also made at Frankford Arsenal during April 1956. This loading used a 180-gr. boattail bullet with GM jacket without cannelure. The cartridge overall length was 2.739 in., and the cases were headstamped [NATO symbol] FA 56 with brass primer and green sealant. The round had a velocity of 2,300 fps and was reportedly designated the T1. The most accurate sub-lots of Frankford Arsenal match cartridges were normally earmarked for the teams that were firing at the various match competitions. The 20-round cartons of these often included an insert with what the Arsenal called a “target card” which was a reproduction of the center of the shot group taken from the in-process test firings showing either the figure of merit (FM) or mean radius (MR).
During September 1956, the T275 International Match cartridge was slightly modified by increasing the velocity to 2,440 fps and changing the bullet seating depth so the cartridge would better chamber in the Swiss Hammerli rifles being used by the U.S. shooting team. This round was designated the T275E1 and was shown on Frankford Arsenal drawing FB 38714 dated September 26, 1956. There is conflicting information as to when the T275E1 cartridge was actually loaded, but it was reportedly made later for the International Matches in 1959. In 1956, a cartridge designated the T275E2 International Match was shown on Frankford Arsenal drawing FB38717 dated October 22, 1956. This round was similar to the T275E1, except it was loaded to a higher velocity of 2,600 fps. The cartridge overall length was approximately 2.950 in. and, according to the drawing, was to be identified by a black primer sealant.
Later in 1956, a change in accuracy requirements resulted in the T275E3 International Match cartridge. This round was loaded to a velocity of 2,640 fps and to an overall length of 2.860 in. The T275E3 was shown on Frankford Arsenal drawing FB39529 dated April 11, 1958. The final T275-series cartridge was designated the T275E4 and was loaded to a lower velocity of 2,275 fps with the same overall length as the T275E3. The designation T275E5 International Match was also assigned in 1960, but there is no evidence that this round ever progressed beyond the design study stage of development. …"
By the way, work on the book, which has 650 pages and 850 Figures (many with multiple items), is progressing well and a 2015 publication is still in sight.