In this Scenario the Trader A buys the shares before the exdate of the reverse split event and will therefore purchase at pre reverse split quantities, ISIN and prices. The shares will settle into his account on the RECORD DATE. The custodian will look at who has the shares at close of business on the RECORD DATE and will affect the payments one day later accordingly (in this case to trader A). There is no trasforming of trades involved.
Example worked out:
Reverse split on ISIN NL00B10RZP78
NEW ISIN: NL0000488639
RATIO: 10:1
Nominal value per share pre-split: EUR 0.05
Nominal value per share post split: EUR 0.50
Market price per share Pre-split: EUR 0.30
Market price per share Post-split: EUR 3.00
Trader A buys 100 shares NL00B10RZP78 from Trader B on Ex-1 of the reverse split. The trades settle on the record date. On record date +1 the custodian will debit the 100 shares NL00B10RZP78 from Trader A's account and credit him with 1 new share NL0000488639. No transformations of the trades is required.