I wonder if the battery tray had been tipped down to allow it to drop out, there's not many fixings on the tray. It does seem to be a bit of a stonker of a battery though. look in the hand book it should tell you of the correct designation. Lots of the classics now use 075 batteries but it's not always been the case.
With regard to the access panels, I knew early cars didn't have them but later did. As far as I remember it evolved from mods made by owners; eventually after being asked by customers to install one at the time of build Peter Morgan made it standard fit.... from the days when Morgan listened to customers and through that many changes were incorporated into the build. Someone here, possibly Dave Wellings, might know when it was standardised. However even very recently there have been requests on this site for measurements in order to install those panels. It's not going to be that ridiculous when you find the solution so don't start going down the line of moving everything. someone managed to squeeze that battery in and as JBK said,I'll bet they didn't go to too much trouble.
