Every Solution Needs a Problem
High Availability is like any other concept only useful in a context that needs such. Here the context is non planned requests at random time - or in other words: Online information/transaction systems.
There were none of these in the stone age of computing, as terminals in common distribution only became common in the late 60s. Without Availability no High Availability :))
It was a very niche niche market.
In addition one needs to understand that the primary solution was to increase reliability first. Even a fully functional mainframe of the 1970 records roughly one non-recoverable memory error per 32 KiB core stack per month. Make that a 1 MiB memory any you'll get a non-recoverable error per day, halting at least the application using that stack.
As a result, fast error recovery was prioritized.
Of course, for highest end applications hot standby systems were used. For example the information system at the 1972 Olympics consist of two /370 compatible mainframes (each with two MiB!) one active, one in hot standby with automated fall over, able to continue operation within minutes.
So yes, stuff like that was already common. Not just with mainframes but as well for communication nodes. Though, at extreme high cost - one had to almost double the investment. Either because of twice the hardware, or specialized dual units that simply sold less thus cost more.
Notable here is that none of the mainframe manufacturers made a big fuzz around, as these measures were simply seen as regular business case. Same way as multiple ways to disk controllers and disks create fail safety.
When looking at minis, some points need to be considered:
- Minis opened computing to way more use cases, thus considerably more installations and thus more visibility - and more information to be found.
- Likewise the niche of fault tolerant systems wasn't so niche anymore, thus more visible.
- In the mini market every little improvement was shouted out like the invention of sliced bread, generating again more visibility.
- Even more, a company like Tandem focused exactly on that niche (*1) will of course make shout out its USP broadly.
Long story short: Yes, those systems existed way before 1974, but all was considered standard business practice.
*1 - IIRC they were HP guys and HP wasn't really interested in the idea of hardware failover at all - too niche even for a mini manufacturer - at least at the time.