Based on the photos of the original, I bet Signature Plastics was the keycap manufacturer. Although the switches aren't MX, it looks like uniform-profile R3 SA. If so, they might have the tooling for original-legend function keys.
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These were not produced by Signature Plastics, they were made by Micro Switch. Comptec (SP has their tooling after a buyout in 2001) did not make these keycaps. I'm actually not sure if Comptec was active in keycaps in the mid 1970s. I was not able to find any activity from them before 1982 when the Wyse and BBC keboards were made. (They were founded 1973 but just did normal injection molding at first).
At first I thought they were the same, but they are actually different if you look closely.
The very top of the keycaps on SP SA has slightly more area than these, and the top is less dished. The top is more rectangular because of this.
The bottom edges are curved (Micro Switch come to a point).
Surface texture is also different
The legends are also different, but this is harder to see.
Fun fact: SP has the stem molds to produce new SA keycaps that fit Micro Switch stems. This makes sense as the profile was designed for older keyboards.
Details about the keyboard: it features Micro Switch SW keyswitches and is from 1975. The part number is 102SW11-1 and I was not able to find any other pictures of it for my Micro Switch photos and data archive.
This keyboard is notable for featuring backlit SW switches which are very uncommon. The output to the terminal is probably "parallel ASCII" which was common for keyboards in the early-to-mid 1970s. The Apple 1 for example could use several different keyboards (one of which was a Micro Switch unit) because it took this input, and other Texas Instruments keyboards used it as well. This is not confirmed but if XMIT posts more photos of the PCB I could determine more information.
I tried to attach some pictures. Sorry for the photo quality, I was not focus stacking my macro photos at the time I took these.
