In the "Way Back When" I used Shilling sized five new pence pieces for bases.
I also used cut down 'phone cards for squarish bases.
A colleague of mine supplied me with some sharpened metal conduit that I used as a punch to get shilling sized bases out of plasticard.
In the 'nineties, I based my 40K on square or round plasticard.
I then started using the slots bases generally.
When 7TV came out, I added a circle of 7mm scale embossed Slater's plasticard on top.
When I started 28mm Bolt Action, I went over to the Renedra 25mm round bases.
Frostgrave and now Oathmark are on the same bases, but my Space Opera (including the future Spacegrave figures) remain on 30mm lipped bases.
As does the Albedo Combat Patrol.
Moderns, Vietnam and Zona Alfa are Renedra 25mm round, but my Walking Dead figures are back on 25mm slotta bases.