Visually there is very little difference between a D7 and a D10 barrel. Except for the little hole for the dowel the port timings and inlet tract are different ad they are designed to run different carbs and pistons. You can swap some parts but not others.

All D10 onwards barrels have a small  hole in the bottom face for the dowel to fit into.

All D10 onwards crankcases will have a dowel in them to match up with the hole in the corresponding barrel. This dowel is easy to remove and does ittle to contribute to the performance of the engine. As Sunny says it does serve as a sanity check but I also think it prevents the barrel from twisting to the right when a high level pipe is fitted...other members will have other theories.

D7 barrels do not have this little hole. The D7 barrel is designed to run in conjunction with the correct D7 top end...carb, piston, head, exhaust etc. The stock D7 has a flat topped piston.

Likewise the D10 barrel is designed to run with the D10 top end. The stock D10 runs a domed piston and a profiled cylinder head.

So....by way of answer....if you remove the dowel from the crancase mouth you can fit a D7 barrell onto a D10 bottom end. But....unless you also fit the rest of the D7 top end it will not run right until you mess about with carbs and compression ratios, ignition timing ⁷etc eventually you will  wish you had never done it in the first place.

Likewise ...you can run a D10 barrel on a D7 bottom end but as above ideally it needs the complete D10 top end, carb, head, piston and engine settings to run right and you also risk early failure of your big-end with all the extra power and higher revs. 

Having tried both in the past and at present I have a D10A with a D7 top end because I cannot get to the engineering workshop to collect my D10 barrel I have fitted the complete D7 top end. With the D7 power delivery and a 4 speed box it makes a delightful and economical  touring bike.Unless you want to get into the realms of carb tuning and trying out different combinations of parts and ignition timings etc I would advise keep it all stock. 

Whatever you do avoid putting a D10 piston into a D7 engine without top end modifications...the stock D7 cylinder head is not compatible with a domed piston.  

I hope this gives you a steer in the right direction. Ride on.