I have a couple of 3550,2950,6509,4507 and trying to find out if they are running L2 or L3
When you configure a device with an "ip default-gateway" you are telling the device to forward all traffic to that gateway. The device itself is not routing, it merely sends all packets to the default-gateway. This is like a desktop PC that has a default-gateway. It sends all it's traffic that is destined for a different subnet to that default-gateway.
A L2 switch or a L3 switch that is only acting as a L2 switch usually has one L3 vlan interface. This interface is not for forwarding client data traffic. It is used to remotely manage the switch. So you can telnet to the L3 vlan interface on the switch. But without a default-gateway on the switch you would not be able to telnet from a different subnet.
A default-route is doing the same thing but in a different way. A default-route again tells the device to send all traffic to the next-hop for any remote subnet. So in that way it is acting in the same way as the default-gateway. But you use a default-route on a device when it makes routing decisions. Sometimes the only route a device has is the default-route but often it has other routes as well. Client PC's generally do not have default-routes.
So you can see a switch with a single L3 vlan interface with a default-route is technically acting as L3 switch ie. it is routing even though it is very similiar to a L2 switch with an ip default-gateway. Like i say, more often than not a switch acting as a L3 switch will have multiple L3 vlan interface and a bigger routing table but as your example shows it doesn't have to.
default-gateway is used on a L2 switch.
default route is used on a L3 switch.
No idea why it was set up this way. If you only have one L3 vlan interface as you do on the config you supplied i would assume it is L2 only.
3550/3560/3750 can be either L2 or L3
4500 - more will probably be L3 than L2 but they can be L2
6500 - even more will be L3 than L2
That is just a rough guideline.