Just kidding. Actually your question made me wonder what service packs were.
On reddit I found that they were, as one person stated, "Depends on the
era, but those sets were featured in the back of larger sized catalogs, LEGO
Shop at Home, LEGO Service catalogs, and if you were in Europe during the early
years toy stores would sell them directly."
Another stated. "You used to get leaflets in the backs of catalogues or
maybe the sets too and you could fill out a form, send it off with a cheque,
and wait a few weeks to receive your cool stuff."
I am sure others here could explain in more detail.
Just kidding. Actually your question made me wonder what service packs were.
On reddit I found that they were, as one person stated, "Depends on the
era, but those sets were featured in the back of larger sized catalogs, LEGO
Shop at Home, LEGO Service catalogs, and if you were in Europe during the early
years toy stores would sell them directly."
Another stated. "You used to get leaflets in the backs of catalogues or
maybe the sets too and you could fill out a form, send it off with a cheque,
and wait a few weeks to receive your cool stuff."
I am sure others here could explain in more detail.