Prefix 正 (しょう) before a number


According to jisho, 正 (しょう) can act as a prefix before a number with meaning
"exactly; precisely", but despite not having any label of "Archaism" or
similar for that meaning precisely, I can't find any example where I can see
used this prefix 正 (しょう) before a number.

In dictionary Goo I have found the following for 正 (しょう):

> ふつう、時間や量を示す語の上に付いて、きっちりその時間や量であることを表す。「正八時」「正一合」

However, I have never heard things like「正八時」when telling the time and I'm not
sure if it is still used nowadays or not, and regarding to「正一合」, it seems it
has to do with sake, but I don't know what does it exactly refer to.

合 seems to be a traditional unit of volume (approx. 0.1804 litres), but in
terms of sake I'm completely lost and I don't have idea of how much capacity
is normally a bottle of sake and if 正一合 is the part of alcohol, another
ingredient, the total product of sake or anything else, and also I don't know
if when ordering sake, you must say something like 正一合の酒お願いします.

**Then, could you please explain me a little bit more the examples「正八時」「正一合」or
tell me an example of when you use or you have seen used this prefix 正 (しょう)
before a number?**




Regarding 正一合, it looks like it is customary for some bars to label sake they
serve in a tokkuri 一合, 二合 etc while the actual volume is close to but slightly
less than that. 正一合 makes it clear that the actual volume is exactly 一合.
Alternatively, bars can label it 180ml to avoid any ambiguity.

References:

  * <https://sake-kikizakeshi-biwa.com/nihonsyu-ichigou/>
  * <https://magazine.asahi-shuzo.co.jp/know/171>

Regarding 正八時, I think it's another way to say 八時ちょうど and 八時ゼロ分, but shorter.
It sounds kind of archaic and rigid for normal conversation, though. It looks
like 正X時半 is possible, too.

Example:
[https://kokkai.ndl.go.jp/#/detail?minId=102204024X02819550614&spkNum=43](https://kokkai.ndl.go.jp/#/detail?minId=102204024X02819550614&spkNum=43)

> それでは本会議の開会時刻は **正一時半** ということにいたします




Regarding 正一合, it looks like it is customary for some bars to label sake they
serve in a tokkuri 一合, 二合 etc while the actual volume is close to but slightly
less than that. 正一合 makes it clear that the actual volume is exactly 一合.
Alternatively, bars can label it 180ml to avoid any ambiguity.

References:

  * <https://sake-kikizakeshi-biwa.com/nihonsyu-ichigou/>
  * <https://magazine.asahi-shuzo.co.jp/know/171>

Regarding 正八時, I think it's another way to say 八時ちょうど and 八時ゼロ分, but shorter.
It sounds kind of archaic and rigid for normal conversation, though. It looks
like 正X時半 is possible, too.

Example:
[https://kokkai.ndl.go.jp/#/detail?minId=102204024X02819550614&spkNum=43](https://kokkai.ndl.go.jp/#/detail?minId=102204024X02819550614&spkNum=43)

> それでは本会議の開会時刻は **正一時半** ということにいたします

