The barrel on my 6.087 is a 4 of 56 with a heat lot of A226B. Doubt I will every own, or much less see, a 6.0 H&R. Tony Giacobbe posted that in 1975, when he got his first M1rifle, it was a HRA M1 D, serial number 6034402. In 1989 he sent it to Scott Duff, who at that time had never seen one.
Not sure of the real reason for allocating these 400 serial numbers. In a 2001 article by Scott Duff, he states that on Aug. 6, 1956, H&R had received an additional contract through the Boston Ordnance District, for 400 rifles. The assigned serial range was from 6034330 through 6034729. Scott further stated " it is believed this was a "clean up" order to allow H&R to use up receivers and other components.". Tony Giacobbe stated that "SA allocated HRA 400 serial numbers in the 6 million range to make up for receivers they made wrong and were destroyed."