Unfamiliar construction from Nineteen Eighty-Four: "...had been used to gather there..."

Solution 1:

This is the past perfect of an archaic form of the verb "used to". Consider this Ngram for "he was used to go". It means "he was accustomed to go", and it was around 20 times as common in 1800 as it is today. My impression is that it was generally used in the past simple or present perfect, and not as much in present tense.

Essentially the same construction can be found in Jane Austen:

"I have been used to consider poetry as the food of love," said Darcy,

She had been used to consult him in every difficulty, and he loved her too well to bear to be denied her confidence now.

So conjugated, it becomes:

The leaders of the party were used to gather there,
The leaders of the party had been used to gather there.

Solution 2:

It should be either:

1) The old, discredited leaders of the Party used to gather there before they were finally purged.

2) The old, discredited leaders of the Party had used to gather there before they were finally purged.

or

3) The old, discredited leaders of the Party had been used to gathering there before they were finally purged. [= 'had been accustomed to gathering']

'Had been used to' seems to be some kind of accidental variant.