Here is the deal on eBay.
I am also disposing of some of my cross use period covers. I believe they are reasonable start prices. The Singapore FDC with Malaya / Malaysia postmarks are harder to find than Malaya / Malaysia FDCs with Singapore postmarks.
Here is the link: https://www.ebay.com/sch/sky52200/m.html?item=362519938515&hash=item5467df45d3%3Ag%3ALAQAAOSwxtVcJe4n%3Ark%3A1%3Apf%3A0&rt=nc&_trksid=p2047675.l2562
To refine, just use “Cross Use”.
For a short history of cross use (these are not for sale): https://myfdc.me/singapore-in-malayamalaysia/
Thank you for your support!