If sand from your filter is blowing back into your pool then something in your filter is broken, if you have the model number I can help a lot better. You say you don't have laterals, and you have a screen. Is there any big holes in the screen or tear or an entrance in to the stand pipe where the sand can go in? Make sure the stand pipe isn't cracked and it goes proper into the multiport as it should.
Some filters have a tube that goes to the stand pipe that attaches to a screen that releases trapped air and if that doesn't have a screen or the tube is gone sand will blow back into your pool.
The sand does wear down after time and is suggested to replace every 5 - 10 years. If it's as old as your filter it could have worn down enough to pass thru your screens. (That's rare thou).
If you can't figure it out 40 years is old for a filter and I would replace it with a cartridge filter. They are very efficient and let's water flow thru them better then sand or de filters. Also they are very easy to clean.
Any questions just ask.