evening all.
That frame is a Montessa Metisse. You can get lots in there. A unit Triumph will go in nicely if you hav ethe time, skill and know how. I have pics of one that late great Pat French did (Mojo helped him, infact I think Mark did the work....)
The pins are two steel rods that point out from the back of the tank and locate into the holes. The plate is brazed in place on some frames and bolted on in others. A tank with no pins should fit fine. That type of tank is held in place by the seat base, the pins give better location, you can see why they did it to stop the tank riding up at the back.
Either tank will fit.
I have steel both types if reqd, but they are readily available on ebay autos in America.
Beautiful alloy ones are available (made by craftsmen and priced accordingly)
The MK4 fibreglass tank will fit that frame and new ones are made to order (Barry, spoke to THE man today, news update to follow)
You can also fit the MK3 style tank with the later underside (special mould has been made to do this) to allow it to go over the frame rails and instantly make the bike look like the earlier versions.
I think that covers it ~ all the bases are covered!