Parts games, addictive and potentially a HUGE waste of time.
D.A.T.A 1963 and 1970 show no GI entries in index. No later editions available.
GI databook 1988 shows lots of GIxxx rectifiers, but no 400 series.
Motorola and GE replacement guides (like NTE but extinct) show a few GIxxx
rectifier entries. 1977 ECG shows nothing of interest. Current NTE listings are also few and incomplete. For some reason replacement lines don’t pick up the GI series of rectifiers. No 400 series.
So, it appears that your part is from the 1970s or early 1980s. I assume that it has no colorband identification so that eliminates the 1960s.
Nothing in datasheetarchive or military databases. Nothing from google search.
A 1N428 is not listed in any old databook. Motorola 1970 has industry-wide rectifier data and it is not listed. Only a 429.
The GI428 was probably made for only a few years. It doesn’t seem to be a private number.
Newark 1983 catalog has GI semiconductors, but no diodes or rectifiers, and the semiconductor index shows no GIxxx.
I thought I had an old GI databook somewhere. If I find it I will post any good results. Mark