ok, So I've re-worked that VM (and updated it to Centos 7), it is now Ext4:
root@pve:~# proxmox-backup-client map vm/103/2021-02-19T16:27:42Z drive-scsi0.img --repository "root@pam@192.168.100.98:pbsbackuplocal"
Password for "root@pam": ********
Image...