TDK SF - 1986 - EU
The rumor is that the SF is 70s SA formulation. When they introduced a new SA formulation in the mid 80s, they used the remaining stocks of the old SA tape for the SF range, which was the "entry level" TDK Type II until the awful CDing2 came along.
Looking at the shell here, and noting its great performance, we'd hazard a guess that this is the earlier generation SA, in a novel blue shell.
Never called the SF in the USA, these are much rarer than the contemporary SA, but in a blind test, you'd be hard pushed to tell the difference. A bit of negative bias and they sound great.
And they look great.
TDK was founded in Tokyo, Japan, on 7 December 1935 to manufacture the iron-based magnetic material ferrite, which had been recently invented by Yogoro Kato and Takeshi Takei. In 1952 and 1957 they began production of magnetic tapes, with compact cassette tapes following in 1966; it is for these that the company is most widely noted. TDK used to manufacture an extensive portfolio of magnetic and optical media, including several formats of videotape and blank CD-R and recordable DVD discs until the recording business was sold to Imation in 2007.
Since 1997 TDK has gradually withdrawn from the production of compact cassettes. First with the MA-X and AR ("Acoustic Response"), then the AD ("Acoustic Dynamic") and SA-X line in 2001 and 2002 respectively, then the MA ("Metal Alloy") line in 2004. The SA ("Super Avilyn") and D ("Dynamic") lines were withdrawn in 2012 under Imation ownership. Industry trends see the company moving into new forms of media; in 2004 TDK was the first media manufacturer to join the companies developing BD post-DVD technology. TDK operated a semiconductor division in California for about a decade, but divested it in 2005.
In late 2007, Imation acquired TDK's recording business, including flash media, optical media, magnetic tape, and accessories, for $300 million. This also included a license to use the "TDK Life on Record" brand on data storage and audio products for 25 years. In September 2015, Imation announced that it had agreed to relinquish this license and would cease selling TDK-branded products by the end of the year.
TDK SF - 1986 - EU