How to get lsn(xid) numbers of transactions from WAL file?
Run pg_waldump on the WAL segment.
Then you will get lines like
rmgr: Heap        len (rec/tot):    115/   115, tx:        564, lsn: 0/01FECDB0, prev 0/01FECC88, desc: HOT_UPDATE off 1 xmax 564 flags 0x60 ; new off 2 xmax 0, blkref #0: rel 1663/16385/1417 blk 0

In there you have tx: 564, lsn: 0/01FECDB0, which is your desired information.