I'm having a nightmare getting some oars for my BR17. I bought a pair of Lahna 10ft from Norfolk Marine, with collars. Turned out that Lahna have changed the diameter of the shaft for the 10 ft (300cm) oars only to 47mm +-1, so 44mm collars won't fit (not with any amount of hot water and soap). Norfolk Marine have (so far) failed to resolve this problem, so I contacted Lahna.
Apparently they have just started producing these and the UK distributors aren't aware of the change, so don't carry 47mm collars. Lahna sold me a pair of 47 mm collars direct from their Finnish factory, which with some force fit the 48 mm oar. Now I come to put them on the boat, the collar makes the oar too wide for the rowlock!
The bottom line is if you are buying oars over 9 ft length, be very sure about the shaft diameter.
I now have to try to source some rowlocks which will accommodate the fat oars, or buy some thinner oars. I'd like to spare others the same inconvenience and expense.