In the old days, a foreign amateur operating in the US, or a US amateur
operating in a foreign country, would use his own call sign, a slant bar,
and the prefix of the country or location in which he is operating, e.g.
"G5ABC/W1".

Somewhere along the way that got changed so that the prefix is now
inserted ahead of the call sign instead of after, e.g. "W1/G5ABC".

When did that rule change go into effect? It must have been sometime
during the 1990's when I was inactive on ham radio ... one of the many
rule changes that surprised me when I got back into it in 2001, such as
the 5 WPM code requirement for Extras for whom it used to be 20 WPM...