Ampex Professional 671 (1979 US)

This professional type I tape from Ampex with the 671 formulation tape was, as its name suggests, intended for professional use. Some rumors claim it was made for governmental projects, but it is a very adequate type I entry level tape for music recordings as well. Our recordings show it has a typical type I sound for the day, comparable to TDK D, and the Maxell UR.

There is very little information on these tapes as they were quite rare, but we did find this article on Reel to Reel Warehouse that pertains to the same 671 forumation: 

"The Ampex 671 DP (double play) tape is a special formulation often found on Ebay and seems to have been sold to the government for a variety of projects. It is a version of the latest and most improved variation of their classic brown oxide formulation produced from the early 1950s with the company’s Ferrosheen process, developed at the time to give the tape a smoother finish and better frequency response than competing tapes. The 671 was an extended range tape with a lower output, suited to slower speeds and is classified as type 2A, for lower output. The formulation is similar to the 556. It is well suited to be used with a noise reduction system.
This unique Double Play formulation has a noticeably different performance and bias requirement to the 641 or 651, making it ideal for 3-3/4 ips. It does not make sense to use such tapes at 15 ips.
As with any thin tape, extra care must be taken when handling as it can easily stretch or crumble under stress.
This is a formula that has withstood the test of time. It is amazing to consider that you can make a high quality recordings on a 60 year old tape, but this is indeed possible.
Tapes of this vintage have a tendency to shed oxide more than modern tapes, although we have found that tapes made by Ampex from this vintage are better than most, especially later batches that presumably adopted the modern production methods of the newer tapes. Keep in mind that oxide shed is not a sign that this tape is degrading, it should be considered normal... They are highly recommended."