O-290G engine cases are like a conical mount, but only have bolt holes bored through, not the conical shapes milled into the mount ears for the rubber cone mounts cushions. Yours may have only the front half modified or simply worn?
O-290 cases are very similar to O-320 E2D and the O-235. That is to say it has 2 separate front main bearings, similar to the other main bearings, and can never be upgraded to run a constant speed propeller. Other O-320s and O-360s have a long single piece front main bearing and more supporting material around this bearing in the case. The right side of these cases have a boss for drilling an oil port. On constant speed propeller equipped engines, this is drilled and has a fitting and hose/pipe from the propeller governor. These long main bearing cased O-320 engines are eligible for Lycoming's upgrade to 160 HP via replacing 7:1 pistons with 8.5:1 pistons. Many home builders have upgraded the 'lesser' main bearing 150 HP O-320 to 8.5:1 pistons without ill effect.
I have an O-290G powered T-18 with O-320 crank, rods, sump, carb, accessory case etc. The O-320 crankshaft has the same stroke as an O-290, but it has a stronger propeller flange (see below). O-290 cylinders, pistons and rings are getting hard to find & expensive, new after market pistons & rings are now relieving that. I'm going the upgrade route for fun. I also have an O-290G case with 'conical' mod so poorly done I consider the case nearly unusable.
I just got a new 'old stock' O-290G case to make into an O-320 by boring the cylinder spigots in the case to accept O-320 cylinders and gain 30 cubic inches. This is an old Airboat mod. the case will fit my T-18 airframe's existing mount. O-290G engines have a solid tappet cam, and the case has special oil troughs next to the tappet bosses that catch oil flung off the crankshaft, then pass that oil thru drilled passages to the push rod tubes where the oil drools down the faces of the heads and carries off heat. Cool old retro-tech. I think O-235s and O-290D aero engines are similar in this but have regular conical mounts.
It's a fun project I use to stay in touch with my several friends in the T-18 group.
You could possibly identify the case as being from an O-290G by looking at the case nose right behind the propeller. O-290G units were attached to generators, so they had a large flange with a ring of bolt holes to mount the generator. Also the crankshaft has a thin flange and did not have the propeller mount bushings. Engines converted to flight use must have the big flange cut off, and you can often see the cut marks, which will look hand filed, maybe hexagonal instead of round cast. Also the thin prop flange may have a pair of half moon re-enforcing doubler plates behind the flange.
Also, O-290 is a 'Narrow Deck' engine. The cylinder flange and bolt pattern is different than later 'Wide Deck' engines. O-290 cylinders (4-7/8" bore cylinders) are held down with regular hex head nuts. When Lycoming increased the bore to 5-1/8" (O-320 & O-360), the margins around the case studs became narrower, so they used barrel shaped nuts with an internal drive (think Allen head) to reduce the nut diameter. You should check, you may be looking at an O-290G. In that case, it's worth nothing because it's being sold by someone who does not know what it is or is mis-representing it. O-290, and O-290G are good engines, but aircraft sales are based on knowledge and trust.
Mine has a sump from an O-320, and the data tag is on the sump, so it has the data tag from the engine it was manufactured with. Misleading unless you read the logbook.
My RV-8 has a narrow deck conical mount O-360 that began life as an IVO-360A1A in a Brantley helicopter. Van's did not stock the motor mount, but when you order it, they have a subcontractor make it in about 2 weeks. In my case, the builder was near, so he called me and I picked it up in person and had a tour of his shop.
Awesome people in aviation!