This cover is addressed to the British stamp dealers, Whitfield King in Ipswich. It was posted on board a steamer on the 18th April 1937 and shows the commemorative cancel for the "10th anniversary of making Nanking the capital of the Nationalist Government" used on the boat. It also bears a c.d.s "Steamer Postmaster No.21" of the same day. The registration marking at top left has been crossed out and indeed only the basic international 25c rate has been paid.
It does not appear to be well known but a number of commemorative cancels were used on board steamers and they are well worth searching for. I pose the question that if cancels could be prepared for use on steamers, why not on the various t.p.o.s on trains? I have never heard of one, but considering much more mail appears from these t.p.o.s than from the river boats, would it not have made sense?