Stationery: 100+50= 150 (piece)

Reason: If B fills all the remaining 50 envelopes, there will be a difference of 150 stationery. However, since A originally put 1 piece of stationery in each envelope, there will be 50 pieces left, so when filling in according to the method of B, there will actually be 200 more pieces of stationery. Since B puts two more than A in each envelope, there will be 200÷2= 100 (envelopes).