In 1957, Valvo-Berichte introduced the original OC81 germanium transistor (first source) which means that any other versions of the part are second sourced (Philips, Mullard, DSI, etc). It has long been a common practice for other manufacturers to offer their own version of parts, sometimes via a license but often just as an alternate work-alike.
Just because a part is second sourced does not mean that it is a fake, as mistakenly posted occasionally in online forums. Many semiconductors are made by companies other than the original producer, and more often than not, perform just as well.
Along that same line, in opamps, a Signetics NE5532 would be first source and the TI 5532 chip would be a second source. The 7660 charge pump is another example and there are numerous copies of it available. Also, the 4558 opamp is made by just about everyone, and no one says they are fake just because the specific 4558 is not produced by the company that first introduced it (Raytheon).
The spec of the OC81d is similar to the AC128, which is sometimes listed as a substitute.