yeah it kinda looks like a squashed "shi" because it's missing the right hand bit: い
in fact I'd as far say that the whole line is outta whack with the rest of the screen. in these old 80's coin ops they would sometimes leave the kanji (raiden for example) but never the instructions. Japanese games from that time rarely have instructions, and oshiateru is not something you'd often see.
インサートコイン works. There's a huge ゲームセンター near me that only has 80's games (I live in downtown tokyo) , I'll have a look next time.