Ignoring the grounding adapter on the top half, what kind of receptacle is this (in a US home built in 1960; this was likely a later addition).
At first I thought it was just a 20A outlet because it has that horizontal notch. But I've never seen the notch on both hot and neutral.
It is an old-style receptacle that is unsafe and needs to be replaced.
See Wikipedia's history of AC sockets, under the section "U.S. combination duplex socket".
Attribution: Author: Robert Casey, Wa2ise at English Wikipedia - Transferred from en.wikipedia to Commons by BPK.
According to the Wikipedia article, the electrical designer Hubble invented this style of receptacle to handle both the USA-typical parallel plugs and an inline form of plug where the pins were turned sideways. (Nowadays we would identify these two kinds of plugs as NEMA 1-15 and NEMA 6-15, see NEMA Connectors, "NEMA Receptacle chart (non-twist lock)")
The danger of these receptacles, as the Wikipedia article points out, is that the T-receptacle allows both modern 15-amp 120V (NEMA 1 & 5) and 20-amp 120V & 240V(NEMA 6) plugs to fit into it (some if a ground pin were removed), and the 20-amp appliances could overload and melt the in-wall wiring faster than the circuit breakers can trip. 240V devices won't work well on 120V circuits. The USA plug/receptacle setup requires that 20-amp plugs not fit in 15-amp receptacles, and uses the pin layouts to control which voltage is provided.
It would be wise to replace it and any of its twins in the house. Check the wiring and circuit breaker for code-compliant gauge and amperage size (14-gauge copper for 15-amp breaker, 12-gauge copper for 20-amp breaker; if the wire is aluminum, research or ask another question). Since the presence of this kind of outlet on the circuit strongly implies there is no safety ground in the circuit, you would want to find the first outlet in the chain of outlets and replace it with a GFCI outlet (input wires from the circuit breaker to the GFCI's LINE screws, the continuing wires to the rest of the circuit go to the GFCI's LOAD screws), then replace the rest of the old no-ground outlets on the circuit with modern NEMA 5-15 outlets. (NEMA 5-15 are 15-amp receptacles, but 15-amp receptacles can go on 20-amp-code-compatible circuits, and there are very few 20-amp 120V devices.) Put "No Equipmnet Ground" stickers on each of the replaced outlets. Research and ask questions on using a GFCI to fix not having a ground.
