Part of the problem is that "Assembly language era" is ill-defined. Personally, I don't think it counts as Assembly language unless you are using a symbolic assembler, because that's what modern programmers think about when you say "assembly".
There is a reasonably common interpretation includes the whole machine code era as "Assembly language", as you are writing it out and then hand-assembling it. Which means the UNIVAC's C-10 machine code counts as "Assembly language", even if they weren't using that terminology. With this interpretation, the "assembly language era" lasted a few years. but I think this inspiration is very misleading to any programmer exposed to a proper assembler.
Anyway, even with my stricter definition, there was an assembly-only era, but it only seems to have existed inside IBM's research labs. They had their first symbolic assemblers running on the "test assembly" by October 1950.
There is very little information about this "test assembly" computer on the internet, doesn't even have a wikipedia page (same with the "Tape Processing Machine" or TPM that followed). But "IBM's early computers" by Charles J Bashe documents it. This computer had not one but two symbolic assemblers running by autumn 1950, which does actually seem to beat most high level languages.
Long before the first 701 was even installed (in April 1953, at IBM's HQ), programmers had already gotten sick of assembly programming, which is why speedcoding was created.
Though, this wasn't just internal IBM programmers. Customers who bought the 701 were given documentation about both the computer and the assembler as early as 1951. These customers had hired programmers, who had started writing assembly code, months before the 701 assembler was even debugged and running on the first prototype 701, and years before they received their computers. So maybe there was also an "Assembly language only era" in the offices of these early 701 customers. But it's kind of an edge case if they didn't have a computer to run the assembler on, or test their programs.
I assume these early programmers were occasionally visiting the prototype 701 to assemble and test their code.
Part of the problem is that "Assembly language era" is ill-defined. Personally, I don't think it counts as Assembly language unless you are using a symbolic assembler, because that's what modern programmers think about when you say "assembly".
There is a reasonably common interpretation includes the whole machine code era as "Assembly language", as you are writing it out and then hand-assembling it. Which means the UNIVAC's C-10 machine code counts as "Assembly language", even if they weren't using that terminology. With this interpretation, the "assembly language era" lasted a few years. but I think this inspiration is very misleading to any programmer exposed to a proper assembler.
Anyway, even with my stricter definition, there was an assembly-only era, but it only seems to have existed inside IBM's research labs. They had their first symbolic assemblers running on the "test assembly" by October 1950.
There is very little information about this "test assembly" computer on the internet, doesn't even have a wikipedia page (same with the "Tape Processing Machine" or TPM that followed). But "IBM's early computers" by Charles J Bashe documents it. This computer had not one but two symbolic assemblers running by autumn 1950, which does actually seem to beat most high level languages.
Long before the first 701 was even installed (in April 1953, at IBM's HQ), programmers had already gotten sick of assembly programming, which is why speedcoding was created.
Though, this wasn't just internal IBM programmers. Customers who bought the 701 were given documentation about both the computer and the assembler as early as 1951. These customers had hired programmers, who had started writing assembly code, months before the 701 assembler was even debugged and running on the first prototype 701, and years before they received their computers. So maybe there was also an "Assembly language only era" in the offices of these early 701 customers. But it's kind of an edge case if they didn't have a computer to run the assembler on, or test their programs.
I assume these early programmers were occasionally visiting the prototype 701 to assemble and test their code.