QUESTION ON LP

If it isn't a 78 it is a mastewr test disc. A few are still around.

I am no expert on 78s but I worked at a record store for 5 years in the late 90's. We had very few 78s and most were classical. In fact if you go to most Half-Price Bookstores (assuming you have these in your area) you will find collections of classical 78s for literally a dime a dozen. Some swing records might be valuable but mostly blues, jazz, and old time country is what is worth the most.

If it is a "78", it is not an LP. LP stands for long-play which at 33.3 rpm provides more than 5 minutes playing time per side (most are 14 to 20 minutes and some examples exceed 20 minutes). 78s have a maximum playing time under 5 minutes. The modern (although still mono) LP was introduced by Columbia in 1948.

Size of the disc does not define the type either. LPs and 78s are found in both 10" and 12" versions.

So, since you state "It's not the normal type of 33 1/3 or 45 rpm" which we may assume means it sounds slow or dragged out, and the disc is heavier and thicker than a normal LP, it is most likely a 78 as others have suggested.

If you only have one of this type, it would be entirely your call whether to keep it or not, but don't expect you found something rare or valuable.

Maybe it is an old laser disc?