Nice relic you have and worthy of a little cleaning. Just as an example here is a before and after of one I cleaned for an on going relic restoration. First I sprayed everything in a good releasing oil letting it soak over night then removed loose material using a brass wire brush. To clean thoroughly you will have to remove the rear site and bolt release catch which will require some form of heat.
Before you attempt to rotate the rear site assembly take note that it originally was held in place with a tiny screw and silver soldered in position. Your barrel appears to have been cut off just in-front of the assembly so perhaps this dislodged it. You will be amazed what survives under that rust. Cleaning this example revealed the MOD 98 mark and serial numbers.