Ron what you have described is the Dave Brown mount In 46 class size and indeed the L stands for longer beams. Because of the hole spacing you can access all four of the firewall bolts from the nose ring with a long driver even with the engine installed. For fixed cowls this is critical.

These are wonderful mounts that work very well in very tight old style fixed cowls or any tight situation. Far preferable over the large square adjustables, I use them almost exclusively up to 120's.

They come with self tapping screws that are sized for the size of mount you purchased. The back of the package lists the proper fractional drill bit size to use for drilling the pilot holes. There is no need to tap the holes and there is no need to file or spot mill flats, In fact that will only serve to weaken the beams considerably.

I know its contrary to popular opinion to actually use the mount as intended but the fact remains the mounts and the method has been around for many years and thats because they work.
Yes I do have even 120's mounted with the screws both a Webra 2 stroke and an OS 4 stroke as well as probably the most violent shaker in the plane of rotation I can imagine YS 110 running on forty percent nitro in some of my warbird racers.

All use the supplied screws an all stay tight. Its a good method give it a chance or you will never reap the benefits.

John